The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom

CHAPTER V. GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC.

13. GERANIACEAE.—Pelargonium zonale.

This plant, as a general rule, is strongly proterandrous, and is therefore adapted for cross-fertilisation by the aid of insects. (5/1. Mr. J. Denny, a great raiser of new varieties of pelargoniums, after stating that this species is proterandrous, adds ‘The Florist and Pomologist’ January 1872 page 11, “there are some varieties, especially those with petals of a pink colour, or which possess a weakly constitution, where the pistil expands as soon as or even before the pollen-bag bursts, and in which also the pistil is frequently short, so when it expands it is smothered as it were by the bursting anthers; these varieties are great seeders, each pip being fertilised by its own pollen. I would instance Christine as an example of this fact.” We have here an interesting case of variability in an important functional point.) Some flowers on a common scarlet variety were self-fertilised, and other flowers were crossed with pollen from another plant; but no sooner had I done so, than I remembered that these plants had been propagated by cuttings from the same stock, and were therefore parts in a strict sense of the same individual. Nevertheless, having made the cross I resolved to save the seeds, which, after germinating on sand, were planted on the opposite sides of three pots. In one pot the quasi-crossed plant was very soon and ever afterwards taller and finer than the self-fertilised. In the two other pots the seedlings on both sides were for a time exactly equal; but when the self-fertilised plants were about 10 inches in height, they surpassed their antagonists by a little, and ever afterwards showed a more decided and increasing advantage; so that the self-fertilised plants, taken altogether, were somewhat superior to the quasi-crossed plants. In this case, as in that of the Origanum, if individuals which have been asexually propagated from the same stock, and which have been long subjected to the same conditions, are crossed, no advantage whatever is gained.

Several flowers on another plant of the same variety were fertilised with pollen from the younger flowers on the same plant, so as to avoid using the old and long-shed pollen from the same flower, as I thought that this latter might be less efficient than fresh pollen. Other flowers on the same plant were crossed with fresh pollen from a plant which, although closely similar, was known to have arisen as a distinct seedling. The self-fertilised seeds germinated rather before the others; but as soon as I got equal pairs they were planted on the opposite sides of four pots.

TABLE 5/49. Pelargonium zonale.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 22 3/8 : 25 5/8. Pot 1 : 19 6/8 : 12 4/8.

Pot 2 : 15 : 19 6/8. Pot 2 : 12 2/8 : 22 3/8.

Pot 3 : 30 5/8 : 19 4/8. Pot 3 : 18 4/8 : 7 4/8.

Pot 4 : 38 : 9 1/8.

Total : 156.50 : 116.38.

When the two lots of seedlings were between 4 and 5 inches in height they were equal, excepting in Pot 4, in which the crossed plant was much the tallest. When between 11 and 14 inches in height, they were measured to the tips of their uppermost leaves; the crossed averaged 13.46, and the self-fertilised 11.07 inches in height, or as 100 to 82. Five months later they were again measured in the same manner, and the results are given in Table 5/49.

The seven crossed plants now averaged 22.35, and the seven self-fertilised 16.62 inches in height, or as 100 to 74. But from the great inequality of the several plants, the result is less trustworthy than in most other cases. In Pot 2 the two self-fertilised plants always had an advantage, except whilst quite young over the two crossed plants.

As I wished to ascertain how these plants would behave during a second growth, they were cut down close to the ground whilst growing freely. The crossed plants now showed their superiority in another way, for only one out of the seven was killed by the operation, whilst three of the self-fertilised plants never recovered. There was, therefore, no use in keeping any of the plants excepting those in Pots 1 and 3; and in the following year the crossed plants in these two pots showed during their second growth nearly the same relative superiority over the self-fertilised plants as before.

Tropaeolum minus.

The flowers are proterandrous, and are manifestly adapted for cross-fertilisation by insects, as shown by Sprengel and Delpino. Twelve flowers on some plants growing out of doors were crossed with pollen from a distinct plant and produced eleven capsules, containing altogether twenty-four good seeds. Eighteen flowers were fertilised with their own pollen and produced only eleven capsules, containing twenty-two good seeds; so that a much larger proportion of the crossed than of the self-fertilised flowers produced capsules, and the crossed capsules contained rather more seed than the self-fertilised in the ratio of 100 to 92. The seeds from the self-fertilised capsules were however the heavier of the two, in the ratio of 100 to 87.

Seeds in an equal state of germination were planted on the opposite sides of four pots, but only the two tallest plants on each side of each pot were measured to the tops of their stems. The pots were placed in the greenhouse, and the plants trained up sticks, so that they ascended to an unusual height. In three of the pots the crossed plants flowered first, but in the fourth at the same time with the self-fertilised. When the seedlings were between 6 and 7 inches in height, the crossed began to show a slight advantage over their opponents. When grown to a considerable height the eight tallest crossed plants averaged 44.43, and the eight tallest self-fertilised plants 37.34 inches, or as 100 to 84. When their growth was completed they were again measured, as shown in Table 5/50.

TABLE 5/50. Tropaeolum minus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 65 : 31. Pot 1 : 50 : 45.

Pot 2 : 69 : 42. Pot 2 : 35 : 45.

Pot 3 : 70 : 50 4/8. Pot 3 : 59 4/8 : 55 4/8.

Pot 4 : 61 4/8 : 37 4/8. Pot 4 : 57 4/8 : 61 4/8.

Total : 467.5 : 368.0.

The eight tallest crossed plants now averaged 58.43, and the eight tallest self-fertilised plants 46 inches in height, or as 100 to 79.

There was also a great difference in the fertility of the two lots which were left uncovered in the greenhouse. On the 17th of September the capsules from all the plants were gathered, and the seeds counted. The crossed plants yielded 243, whilst the same number of self-fertilised plants yielded only 155 seeds, or as 100 to 64.

Limnanthes douglasii.

Several flowers were crossed and self-fertilised in the usual manner, but there was no marked difference in the number of seeds which they yielded. A vast number of spontaneously self-fertilised capsules were also produced under the net. Seedlings were raised in five pots from the above seeds, and when the crossed were about 3 inches in height they showed a slight advantage over the self-fertilised. When double this height, the sixteen crossed and sixteen self-fertilised plants were measured to the tips of their leaves; the former averaged 7.3 inches, and the self-fertilised 6.07 inches in height, or as 100 to 83. In all the pots, excepting 4, a crossed plant flowered before any one of the self-fertilised plants. The plants, when fully grown, were again measured to the summits of their ripe capsules, with the result in Table 5/51.

TABLE 5/51. Limnanthes douglasii.

Heights of plants to the summits of their ripe capsules, measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 17 7/8 : 15 1/8. Pot 1 : 17 6/8 : 16 4/8. Pot 1 : 13 : 11.

Pot 2 : 20 : 14 4/8. Pot 2 : 22 : 15 6/8. Pot 2 : 21 : 16 1/8. Pot 2 : 18 4/8 : 17.

Pot 3 : 15 6/8 : 11 4/8. Pot 3 : 17 2/8 : 10 4/8. Pot 3 : 14 : 0.

Pot 4 : 20 4/8 : 13 4/8. Pot 4 : 14 : 13. Pot 4 : 18 : 12 2/8.

Pot 5 : 17 : 14 2/8. Pot 5 : 18 5/8 : 14 1/8. Pot 5 : 14 2/8 : 12 5/8.

Total : 279.50 : 207.75.

The sixteen crossed plants now averaged 17.46, and the fifteen (for one had died) self-fertilised plants 13.85 inches in height, or as 100 to 79. Mr. Galton considers that a higher ratio would be fairer, namely, 100 to 76. He made a graphical representation of the above measurements, and adds the words “very good” to the curvature thus formed. Both lots of plants produced an abundance of seed-capsules, and, as far as could be judged by the eye, there was no difference in their fertility.]

14. LEGUMINOSAE.

In this family I experimented on the following six genera, Lupinus, Phaseolus, Lathyrus, Pisum, Sarothamnus, and Ononis.

[Lupinus luteus. (5/2. The structure of the flowers of this plant, and their manner of fertilisation, have been described by H. Muller ‘Befruchtung’ etc. page 243. The flowers do not secrete free nectar, and bees generally visit them for their pollen. Mr. Farrer, however, remarks ‘Nature’ 1872 page 499, that “there is a cavity at the back and base of the vexillum, in which I have not been able to find nectar. But the bees, which constantly visit these flowers, certainly go to this cavity for what they want, and not to the staminal tube.”)

A few flowers were crossed with pollen from a distinct plant, but owing to the unfavourable season only two crossed seeds were produced. Nine seeds were saved from flowers spontaneously self-fertilised under a net, on the same plant which yielded the two crossed seeds. One of these crossed seeds was sown in a pot with two self-fertilised seeds on the opposite side; the latter came up between two and three days before the crossed seed. The second crossed seed was sown in like manner with two self-fertilised seeds on the opposite side; these latter also came up about a day before the crossed one. In both pots, therefore, the crossed seedlings from germinating later, were at first completely beaten by the self-fertilised; nevertheless, this state of things was afterwards completely reversed. The seeds were sown late in the autumn, and the pots, which were much too small, were kept in the greenhouse. The plants in consequence grew badly, and the self-fertilised suffered most in both pots. The two crossed plants when in flower during the following spring were 9 inches in height; one of the self-fertilised plants was 8, and the three others only 3 inches in height, being thus mere dwarfs. The two crossed plants produced thirteen pods, whilst the four self-fertilised plants produced only a single one. Some other self-fertilised plants which had been raised separately in larger pots produced several spontaneously self-fertilised pods under a net, and seeds from these were used in the following experiment.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The spontaneously self-fertilised seeds just mentioned, and crossed seeds obtained by intercrossing the two crossed plants of the last generation, after germinating on sand, were planted in pairs on the opposite sides of three large pots. When the seedlings were only 4 inches in height, the crossed had a slight advantage over their opponents. When grown to their full height, every one of the crossed plants exceeded its opponent in height. Nevertheless the self-fertilised plants in all three pots flowered before the crossed! The measurements are given in Table 5/52.

TABLE 5/52. Lupinus luteus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 33 2/8 : 24 4/8. Pot 1 : 30 4/8 : 18 4/8. Pot 1 : 30 : 28.

Pot 2 : 29 4/8 : 26. Pot 2 : 30 : 25.

Pot 3 : 30 4/8 : 28. Pot 3 : 31 : 27 2/8. Pot 3 : 31 4/8 : 24 4/8.

Total : 246.25 : 201.75.

The eight crossed plants here average 30.78, and the eight self-fertilised 25.21 inches in height; or as 100 to 82. These plants were left uncovered in the greenhouse to set their pods, but they produced very few good ones, perhaps in part owing to few bees visiting them. The crossed plants produced nine pods, containing on an average 3.4 seeds, and the self-fertilised plants seven pods, containing on an average 3 seeds, so that the seeds from an equal number of plants were as 100 to 88.

Two other crossed seedlings, each with two self-fertilised seedlings on the opposite sides of the same large pot, were turned out of their pots early in the season, without being disturbed, into open ground of good quality. They were thus subjected to but little competition with one another, in comparison with the plants in the above three pots. In the autumn the two crossed plants were about 3 inches taller than the four self-fertilised plants; they looked also more vigorous and produced many more pods.

Two other crossed and self-fertilised seeds of the same lot, after germinating on sand, were planted on the opposite sides of a large pot, in which a Calceolaria had long been growing, and were therefore exposed to unfavourable conditions: the two crossed plants ultimately attained a height of 20 1/2 and 20 inches, whilst the two self-fertilised were only 18 and 9 1/2 inches high.

Lupinus pilosus.

From a series of accidents I was again unfortunate in obtaining a sufficient number of crossed seedlings; and the following results would not be worth giving, did they not strictly accord with those just given with respect to Lupinus luteus. I raised at first only a single crossed seedling, which was placed in competition with two self-fertilised ones on the opposite side of the same pot. These plants, without being disturbed, were soon afterwards turned into the open ground. By the autumn the crossed plant had grown to so large a size that it almost smothered the two self-fertilised plants, which were mere dwarfs; and the latter died without maturing a single pod. Several self-fertilised seeds had been planted at the same time separately in the open ground; and the two tallest of these were 33 and 32 inches, whereas the one crossed plant was 38 inches in height. This latter plant also produced many more pods than did any one of the self-fertilised plants, although growing separately. A few flowers on the one crossed plant were crossed with pollen from one of the self-fertilised plants, for I had no other crossed plant from which to obtain pollen. One of the self-fertilised plants having been covered by a net produced plenty of spontaneously self-fertilised pods.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

From crossed and self-fertilised seeds obtained in the manner just described, I succeeded in raising to maturity only a pair of plants, which were kept in a pot in the greenhouse. The crossed plant grew to a height of 33 inches, and the self-fertilised to that of 26 1/2 inches. The former produced, whilst still kept in the greenhouse, eight pods, containing on an average 2.77 seeds; and the latter only two pods, containing on an average 2.5 seeds. The average height of the two crossed plants of the two generations taken together was 35.5, and that of the three self-fertilised plants of the same two generations 30.5; or as 100 to 86. (5/3. We here see that both Lupinus luteus and pilosus seed freely when insects are excluded; but Mr. Swale, of Christchurch, in New Zealand, informs me ‘Gardeners’ Chronicle’ 1858 page 828, that the garden varieties of the lupine are not there visited by any bees, and that they seed less freely than any other introduced leguminous plant, with the exception of red clover. He adds “I have, for amusement, during the summer, released the stamens with a pin, and a pod of seed has always rewarded me for my trouble, the adjoining flowers not so served having all proved blind.” I do not know to what species this statement refers.)

Phaseolus multiflorus.

This plant, the scarlet-runner of English gardeners and the Phaseolus coccineus of Lamarck, originally came from Mexico, as I am informed by Mr. Bentham. The flowers are so constructed that hive and humble-bees, which visit them incessantly, almost always alight on the left wing-petal, as they can best suck the nectar from this side. Their weight and movements depress the petal, and this causes the stigma to protrude from the spirally-wound keel, and a brush of hairs round the stigma pushes out the pollen before it. The pollen adheres to the head or proboscis of the bee which is at work, and is thus placed either on the stigma of the same flower, or is carried to another flower. (5/4. The flowers have been described by Delpino, and in an admirable manner by Mr. Farrer in the ‘Annals and Magazine of Natural History’ volume 2 4th series October 1868 page 256. My son Francis has explained ‘Nature’ January 8, 1874 page 189, the use of one peculiarity in their structure, namely, a little vertical projection on the single free stamen near its base, which seems placed as if to guard the entrance into the two nectar-holes in the staminal sheath. He shows that this projection prevents the bees reaching the nectar, unless they go to the left side of the flower, and it is absolutely necessary for cross-fertilisation that they should alight on the left wing-petal.) Several years ago I covered some plants under a large net, and these produced on one occasion about one-third, and on another occasion about one-eighth, of the number of pods which the same number of uncovered plants growing close alongside produced. (5/5. ‘Gardeners’ Chronicle’ 1857 page 725 and more especially ibid 1858 page 828. Also ‘Annals and Magazine of Natural History’ 3rd series volume 2 1858 page 462.) This lessened fertility was not caused by any injury from the net, as I moved the wing-petals of several protected flowers, in the same manner as bees do, and these produced remarkably fine pods. When the net was taken off, the flowers were immediately visited by bees, and it was interesting to observe how quickly the plants became covered with young pods. As the flowers are much frequented by Thrips, the self-fertilisation of most of the flowers under the net may have been due to the action of these minute insects. Dr. Ogle likewise covered up a large portion of a plant, and “out of a vast number of blossoms thus protected not a single one produced a pod, while the unprotected blossoms were for the most part fruitful.” Mr. Belt gives a more curious case; this plant grows well and flowers in Nicaragua; but as none of the native bees visit the flowers, not a single pod is ever produced. (5/6. Dr. Ogle ‘Popular Science Review’ 1870 page 168. Mr. Belt ‘The Naturalist in Nicaragua’ 1874 page 70. The latter author gives a case ‘Nature’ 1875 page 26, of a late crop of Phaseolus multiflorus near London which “was rendered barren” by the humble-bees cutting, as they frequently do, holes at the bases of the flowers instead of entering them in the proper manner.)

From the facts now given we may feel nearly sure that individuals of the same variety or of different varieties, if growing near each other and in flower at the same time, would intercross; but I cannot myself advance any direct evidence of such an occurrence, as only a single variety is commonly cultivated in England. I have, however, received an account from the Reverend W.A. Leighton, that plants raised by him from ordinary seed produced seeds differing in an extraordinary manner in colour and shape, leading to the belief that their parents must have been crossed. In France M. Fermond more than once planted close together varieties which ordinarily come true and which bear differently coloured flowers and seeds; and the offspring thus raised varied so greatly that there could hardly be a doubt that they had intercrossed. (5/7. ‘Fécondation chez les Végétaux’ 1859 pages 34-40. He adds that M. Villiers has described a spontaneous hybrid, which he calls Phaseolus coccineus hybridus, in the ‘Annales de la Soc. R. de Horticulture’ June 1844.) On the other hand, Professor H. Hoffman does not believe in the natural crossing of the varieties; for although seedlings raised from two varieties growing close together produced plants which yielded seeds of a mixed character, he found that this likewise occurred with plants separated by a space of from 40 to 150 paces from any other variety; he therefore attributes the mixed character of the seed to spontaneous variability. (5/8. ‘Bestimmung des Werthes von Species und Varietat’ 1869 pages 47-72.) But the above distance would be very far from sufficient to prevent intercrossing: cabbages have been known to cross at several times this distance; and the careful Gartner gives many instances of plants growing at from 600 to 800 yards apart fertilising one another. (5/9. ‘Kenntnis der Befruchtung’ 1844 pages 573, 577.) Professor Hoffman even maintains that the flowers of the kidney-bean are specially adapted for self-fertilisation. He enclosed several flowers in bags; and as the buds often dropped off, he attributes the partial sterility of these flowers to the injurious effects of the bags, and not to the exclusion of insects. But the only safe method of experimenting is to cover up a whole plant, which then never suffers.

Self-fertilised seeds were obtained by moving up and down in the same manner as bees do the wing-petals of flowers protected by a net; and crossed seeds were obtained by crossing two of the plants under the same net. The seeds after germinating on sand were planted on the opposite sides of two large pots, and equal-sized sticks were given them to twine up. When 8 inches in height, the plants on the two sides were equal. The crossed plants flowered before the self-fertilised in both pots. As soon as one of each pair had grown to the summit of its stick both were measured.

TABLE 5/53. Phaseolus multiflorus.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 87 : 84 6/8. Pot 1 : 88 : 87. Pot 1 : 82 4/8 : 76.

Pot 2 : 90 : 76 4/8. Pot 2 : 82 4/8 : 87 4/8.

Total : 430.00 : 411.75.

The average height of the five crossed plants is 86 inches, and that of the five self-fertilised plants 82.35; or as 100 to 96. The pots were kept in the greenhouse, and there was little or no difference in the fertility of the two lots. Therefore as far as these few observations serve, the advantage gained by a cross is very small.

Phaseolus vulgaris.

With respect to this species, I merely ascertained that the flowers were highly fertile when insects were excluded, as indeed must be the case, for the plants are often forced during the winter when no insects are present. Some plants of two varieties (namely Canterbury and Fulmer’s Forcing Bean) were covered with a net, and they seemed to produce as many pods, containing as many beans, as some uncovered plants growing alongside; but neither the pods nor the beans were actually counted. This difference in self-fertility between Phaseolus vulgaris and multifloris is remarkable, as these two species are so closely related that Linnaeus thought that they formed one. When the varieties of Phaseolus vulgaris grow near one another in the open ground, they sometimes cross largely, notwithstanding their capacity for self-fertilisation. Mr. Coe has given me a remarkable instance of this fact with respect to the negro and a white-seeded and a brown-seeded variety, which were all grown together. The diversity of character in the seedlings of the second generation raised by me from his plants was wonderful. I could add other analogous cases, and the fact is well-known to gardeners. (5/10. I have given Mr. Coe’s case in the ‘Gardeners’ Chronicle’ 1858 page 829. See also for another case ibid page 845.)

Lathyrus odoratus.

Almost everyone who has studied the structure of papilionaceous flowers has been convinced that they are specially adapted for cross-fertilisation, although many of the species are likewise capable of self-fertilisation. The case therefore of Lathyrus odoratus or the sweet-pea is curious, for in this country it seems invariably to fertilise itself. I conclude that this is so, as five varieties, differing greatly in the colour of their flowers but in no other respect, are commonly sold and come true; yet on inquiry from two great raisers of seed for sale, I find that they take no precautions to insure purity—the five varieties being habitually grown close together. (5/11. See Mr. W. Earley in ‘Nature’ 1872 page 242, to the same effect. He once, however, saw bees visiting the flowers, and supposed that on this occasion they would have been intercrossed.) I have myself purposely made similar trials with the same result. Although the varieties always come true, yet, as we shall presently see, one of the five well-known varieties occasionally gives birth to another, which exhibits all its usual characters. Owing to this curious fact, and to the darker-coloured varieties being the most productive, these increase, to the exclusion of the others, as I was informed by the late Mr. Masters, if there be no selection.

In order to ascertain what would be the effect of crossing two varieties, some flowers on the Purple sweet-pea, which has a dark reddish-purple standard-petal with violet-coloured wing-petals and keel, were castrated whilst very young, and were fertilised with pollen of the Painted Lady. This latter variety has a pale cherry-coloured standard, with almost white wings and keel. On two occasions I raised from a flower thus crossed plants perfectly resembling both parent-forms; but the greater number resembled the paternal variety. So perfect was the resemblance, that I should have suspected some mistake in the label, had not the plants, which were at first identical in appearance with the father or Painted Lady, later in the season produced flowers blotched and streaked with dark purple. This is an interesting example of partial reversion in the same individual plant as it grows older. The purple-flowered plants were thrown away, as they might possibly have been the product of the accidental self-fertilisation of the mother-plant, owing to the castration not having been effectual. But the plants which resembled in the colour of their flowers the paternal variety or Painted Lady were preserved, and their seeds saved. Next summer many plants were raised from these seeds, and they generally resembled their grandfather the Painted Lady, but most of them had their wing-petals streaked and stained with dark pink; and a few had pale purple wings with the standard of a darker crimson than is natural to the Painted Lady, so that they formed a new sub-variety. Amongst these plants a single one appeared having purple flowers like those of the grandmother, but with the petals slightly streaked with a paler tint: this was thrown away. Seeds were again saved from the foregoing plants, and the seedlings thus raised still resembled the Painted Lady, or great-grandfather; but they now varied much, the standard petal varying from pale to dark red, in a few instances with blotches of white; and the wing-petals varied from nearly white to purple, the keel being in all nearly white.

As no variability of this kind can be detected in plants raised from seeds, the parents of which have grown during many successive generations in close proximity, we may infer that they cannot have intercrossed. What does occasionally occur is that in a row of plants raised from seeds of one variety, another variety true of its kind appears; for instance, in a long row of Scarlets (the seeds of which had been carefully gathered from Scarlets for the sake of this experiment) two Purples and one Painted Lady appeared. Seeds from these three aberrant plants were saved and sown in separate beds. The seedlings from both the Purples were chiefly Purples, but with some Painted Ladies and some Scarlets. The seedlings from the aberrant Painted Lady were chiefly Painted Ladies with some Scarlets. Each variety, whatever its parentage may have been, retained all its characters perfect, and there was no streaking or blotching of the colours, as in the foregoing plants of crossed origin. Another variety, however, is often sold, which is striped and blotched with dark purple; and this is probably of crossed origin, for I found, as well as Mr. Masters, that it did not transmit its characters at all truly.

From the evidence now given, we may conclude that the varieties of the sweet-pea rarely or never intercross in this country; and this is a highly remarkable fact, considering, firstly, the general structure of the flowers; secondly, the large quantity of pollen produced, far more than is requisite for self-fertilisation; and thirdly, the occasional visit of insects. That insects should sometimes fail to cross-fertilise the flowers is intelligible, for I have thrice seen humble-bees of two kinds, as well as hive-bees, sucking the nectar, and they did not depress the keel-petals so as to expose the anthers and stigma; they were therefore quite inefficient for fertilising the flowers. One of these bees, namely, Bombus lapidarius, stood on one side at the base of the standard and inserted its proboscis beneath the single separate stamen, as I afterwards ascertained by opening the flower and finding this stamen prised up. Bees are forced to act in this manner from the slit in the staminal tube being closely covered by the broad membranous margin of the single stamen, and from the tube not being perforated by nectar-passages. On the other hand, in the three British species of Lathyrus which I have examined, and in the allied genus Vicia, two nectar-passages are present. Therefore British bees might well be puzzled how to act in the case of the sweet-pea. I may add that the staminal tube of another exotic species, Lathyrus grandiflorus, is not perforated by nectar-passages, and this species has rarely set any pods in my garden, unless the wing-petals were moved up and down, in the same manner as bees ought to do; and then pods were generally formed, but from some cause often dropped off afterwards. One of my sons caught an elephant sphinx-moth whilst visiting the flowers of the sweet-pea, but this insect would not depress the wing-petals and keel. On the other hand, I have seen on one occasion hive-bees, and two or three occasions the Megachile willughbiella in the act of depressing the keel; and these bees had the under sides of their bodies thickly covered with pollen, and could not thus fail to carry pollen from one flower to the stigma of another. Why then do not the varieties occasionally intercross, though this would not often happen, as insects so rarely act in an efficient manner? The fact cannot, as it appears, be explained by the flowers being self-fertilised at a very early age; for although nectar is sometimes secreted and pollen adheres to the viscid stigma before the flowers are fully expanded, yet in five young flowers which were examined by me the pollen-tubes were not exserted. Whatever the cause may be, we may conclude, that in England the varieties never or very rarely intercross. But it does not follow from this, that they would not be cross by the aid of other and larger insects in their native country, which in botanical works is said to be the south of Europe and the East Indies. Accordingly I wrote to Professor Delpino, in Florence, and he informs me “that it is the fixed opinion of gardeners there that the varieties do intercross, and that they cannot be preserved pure unless they are sown separately.”

It follows also from the foregoing facts that the several varieties of the sweet-pea must have propagated themselves in England by self-fertilisation for very many generations, since the time when each new variety first appeared. From the analogy of the plants of Mimulus and Ipomoea, which had been self-fertilised for several generations, and from trials previously made with the common pea, which is in nearly the same state as the sweet-pea, it appeared to me very improbable that a cross between the individuals of the same variety would benefit the offspring. A cross of this kind was therefore not tried, which I now regret. But some flowers of the Painted Lady, castrated at an early age, were fertilised with pollen from the Purple sweet-pea; and it should be remembered that these varieties differ in nothing except in the colour of their flowers. The cross was manifestly effectual (though only two seeds were obtained), as was shown by the two seedlings, when they flowered, closely resembling their father, the Purple pea, excepting that they were a little lighter coloured, with their keels slightly streaked with pale purple. Seeds from flowers spontaneously self-fertilised under a net were at the same time saved from the same mother-plant, the Painted Lady. These seeds unfortunately did not germinate on sand at the same time with the crossed seeds, so that they could not be planted simultaneously. One of the two crossed seeds in a state of germination was planted in a pot (Number 1) in which a self-fertilised seed in the same state had been planted four days before, so that this latter seedling had a great advantage over the crossed one. In Pot 2 the other crossed seed was planted two days before a self-fertilised one; so that here the crossed seedling had a considerable advantage over the self-fertilised one. But this crossed seedling had its summit gnawed off by a slug, and was in consequence for a time quite beaten by the self-fertilised plant. Nevertheless I allowed it to remain, and so great was its constitutional vigour that it ultimately beat its uninjured self-fertilised rival. When all four plants were almost fully grown they were measured, as here shown:—

TABLE 5/54. Lathyrus odoratus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 80 : 64 4/8.

Pot 2 : 78 4/8 : 63.

Total : 158.5 : 127.5.

The two crossed plants here average 79.25, and the two self-fertilised 63.75 inches in height, or as 100 to 80. Six flowers on these two crossed plants were reciprocally crossed with pollen from the other plant, and the six pods thus produced contained on an average six peas, with a maximum in one of seven. Eighteen spontaneously self-fertilised pods from the Painted Lady, which, as already stated, had no doubt been self-fertilised for many previous generations, contained on an average only 3.93 peas, with a maximum in one of five peas; so that the number of peas in the crossed and self-fertilised pods was as 100 to 65. The self-fertilised peas were, however, quite as heavy as those from the crossed pods. From these two lots of seeds, the plants of the next generation were raised.

PLANTS OF THE SECOND GENERATION.

Many of the self-fertilised peas just referred to germinated on sand before any of the crossed ones, and were rejected. As soon as I got equal pairs, they were planted on the opposite sides of two large pots, which were kept in the greenhouse. The seedlings thus raised were the grandchildren of the Painted Lady, which was first crossed by the Purple variety. When the two lots were from 4 to 6 inches in height there was no difference between them. Nor was there any marked difference in the period of their flowering. When fully grown they were measured, as follows:—

TABLE 5/55. Lathyrus odoratus (Second Generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Seedlings from Plants Crossed during the two previous Generations.

Column 3: Seedlings from Plants Self-fertilised during many previous Generations.

Pot 1 : 72 4/8 : 57 4/8. Pot 1 : 71 : 67. Pot 1 : 52 2/8 : 56 2/8.

Pot 2 : 81 4/8 : 66 2/8. Pot 2 : 45 2/8 : 38 7/8. Pot 2 : 55 : 46.

Total : 377.50 : 331.86.

The average height of the six crossed plants is here 62.91, and that of the six self-fertilised 55.31 inches; or as 100 to 88. There was not much difference in the fertility of the two lots; the crossed plants having produced in the greenhouse thirty-five pods, and the self-fertilised thirty-two pods.

Seeds were saved from the self-fertilised flowers on these two lots of plants, for the sake of ascertaining whether the seedlings thus raised would inherit any difference in growth or vigour. It must therefore be understood that both lots in the following trial are plants of self-fertilised parentage; but that in the one lot the plants were the children of plants which had been crossed during two previous generations, having been before that self-fertilised for many generations; and that in the other lot they were the children of plants which had not been crossed for very many previous generations. The seeds germinated on sand and were planted in pairs on the opposite sides of four pots. They were measured, when fully grown, with the following result:—

TABLE 5/56. Lathyrus odoratus.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Self-fertilised Plants from Crossed Plants.

Column 3: Self-fertilised Plants from Self-fertilised Plants.

Pot 1 : 72 : 65. Pot 1 : 72 : 61 4/8.

Pot 2 : 58 : 64. Pot 2 : 68 : 68 2/8. Pot 2 : 72 4/8 : 56 4/8.

Pot 3 : 81 : 60 2/8.

Pot 4 : 77 4/8 : 76 4/8.

Total : 501 : 452.

The average height of the seven self-fertilised plants, the offspring of crossed plants, is 71.57, and that of the seven self-fertilised plants, the offspring of self-fertilised plants, is 64.57; or as 100 to 90. The self-fertilised plants from the self-fertilised produced rather more pods—namely, thirty-six—than the self-fertilised plants from the crossed, for these produced only thirty-one pods.

A few seeds of the same two lots were sown in the opposite corners of a large box in which a Brugmansia had long been growing, and in which the soil was so exhausted that seeds of Ipomoea purpurea would hardly vegetate; yet the two plants of the sweet-pea which were raised flourished well. For a long time the self-fertilised plant from the self-fertilised beat the self-fertilised plant from the crossed plant; the former flowered first, and was at one time 77 1/2 inches, whilst the latter was only 68 1/2 in height; but ultimately the plant from the previous cross showed its superiority and attained a height of 108 1/2 inches, whilst the other was only 95 inches. I also sowed some of the same two lots of seeds in poor soil in a shady place in a shrubbery. Here again the self-fertilised plants from the self-fertilised for a long time exceeded considerably in height those from the previously crossed plants; and this may probably be attributed, in the present as in the last case, to these seeds having germinated rather sooner than those from the crossed plants; but at the close of the season the tallest of the self-fertilised plants from the crossed plants was 30 inches, whilst the tallest of the self-fertilised from the self-fertilised was 29 3/8 inches in height.

From the various facts now given we see that plants derived from a cross between two varieties of the sweet-pea, which differ in no respect except in the colour of their flowers, exceed considerably in height the offspring from self-fertilised plants, both in the first and second generations. The crossed plants also transmit their superiority in height and vigour to their self-fertilised offspring.

Pisum sativum.

The common pea is perfectly fertile when its flowers are protected from the visits of insects; I ascertained this with two or three different varieties, as did Dr. Ogle with another. But the flowers are likewise adapted for cross-fertilisation; Mr. Farrer specifies the following points, namely: “The open blossom displaying itself in the most attractive and convenient position for insects; the conspicuous vexillum; the wings forming an alighting place; the attachment of the wings to the keel, by which any body pressing on the former must press down the latter; the staminal tube enclosing nectar, and affording by means of its partially free stamen with apertures on each side of its base an open passage to an insect seeking the nectar; the moist and sticky pollen placed just where it will be swept out of the apex of the keel against the entering insect; the stiff elastic style so placed that on a pressure being applied to the keel it will be pushed upwards out of the keel; the hairs on the style placed on that side of the style only on which there is space for the pollen, and in such a direction as to sweep it out; and the stigma so placed as to meet an entering insect,—all these become correlated parts of one elaborate mechanism, if we suppose that the fertilisation of these flowers is effected by the carriage of pollen from one to the other.” (5/12. ‘Nature’ October 10, 1872 page 479. Hermann Muller gives an elaborate description of the flowers ‘Befruchtung’ etc. page 247.) Notwithstanding these manifest provisions for cross-fertilisation, varieties which have been cultivated for very many successive generations in close proximity, although flowering at the same time, remain pure. I have elsewhere given evidence on this head, and if required could give more. (5/13. ‘Variation of Animals and Plants under Domestication’ chapter 9 2nd edition volume 1 page 348.) There can hardly be a doubt that some of Knight’s varieties, which were originally produced by an artificial cross and were very vigorous, lasted for at least sixty years, and during all these years were self-fertilised; for had it been otherwise, they would not have kept true, as the several varieties are generally grown near together. Most of the varieties, however, endure for a shorter period; and this may be in part due to their weakness of constitution from long-continued self-fertilisation.

It is remarkable, considering that the flowers secrete much nectar and afford much pollen, how seldom they are visited by insects either in England, or, as H. Muller remarks, in North Germany. I have observed the flowers for the last thirty years, and in all this time have only thrice seen bees of the proper kind at work (one of them being Bombus muscorum), such as were sufficiently powerful to depress the keel, so as to get the undersides of their bodies dusted with pollen. These bees visited several flowers, and could hardly have failed to cross-fertilise them. Hive-bees and other small kinds sometimes collect pollen from old and already fertilised flowers, but this is of no account. The rarity of the visits of efficient bees to this exotic plant is, I believe, the chief cause of the varieties so seldom intercrossing. That a cross does occasionally take place, as might be expected from what has just been stated, is certain, from the recorded cases of the direct action of the pollen of one variety on the seed-coats of another. (5/14. ‘Variation of Animals and Plants under Domestication’ chapter 11 2nd edition volume 1 page 428.) The late Mr. Masters, who particularly attended to the raising of new varieties of peas, was convinced that some of them had originated from accidental crosses. But as such crosses are rare, the old varieties would not often be thus deteriorated, more especially as plants departing from the proper type are generally rejected by those who collect seed for sale. There is another cause which probably tends to render cross-fertilisation rare, namely, the early age at which the pollen-tubes are exserted; eight flowers not fully expanded were examined, and in seven of these the pollen-tubes were in this state; but they had not as yet penetrated the stigma. Although so few insects visit the flowers of the pea in this country or in North Germany, and although the anthers seem here to open abnormally soon, it does not follow that the species in its native country would be thus circumstanced.

Owing to the varieties having been self-fertilised for many generations, and to their having been subjected in each generation to nearly the same conditions (as will be explained in a future chapter) I did not expect that a cross between two such plants would benefit the offspring; and so it proved on trial. In 1867 I covered up several plants of the Early Emperor pea, which was not then a very new variety, so that it must already have been propagated by self-fertilisation for at least a dozen generations. Some flowers were crossed with pollen from a distinct plant growing in the same row, and others were allowed to fertilise themselves under a net. The two lots of seeds thus obtained were sown on opposite sides of two large pots, but only four pairs came up at the same time. The pots were kept in the greenhouse. The seedlings of both lots when between 6 and 7 inches in height were equal. When nearly full-grown they were measured, as in Table 5/57.

TABLE 5/57. Pisum sativum.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 35 : 29 6/8.

Pot 2 : 31 4/8 : 51. Pot 2 : 35 : 45. Pot 2 : 37 : 33.

Total : 138.50 : 158.75.

The average height of the four crossed plants is here 34.62, and that of the four self-fertilised plants 39.68, or as 100 to 115. So that the crossed plants, far from beating the self-fertilised, were completely beaten by them.

There can be no doubt that the result would have been widely different, if any two varieties out of the numberless ones which exist had been crossed. Notwithstanding that both had been self-fertilised for many previous generations, each would almost certainly have possessed its own peculiar constitution; and this degree of differentiation would have been sufficient to make a cross highly beneficial. I have spoken thus confidently of the benefit which would have been derived from crossing any two varieties of the pea from the following facts: Andrew Knight in speaking of the results of crossing reciprocally very tall and short varieties, says, “I had in this experiment a striking instance of the stimulative effects of crossing the breeds; for the smallest variety, whose height rarely exceeded 2 feet, was increased to 6 feet; whilst the height of the large and luxuriant kind was very little diminished.” (5/15. ‘Philosophical Transactions’ 1799 page 200.) Recently Mr. Laxton has made numerous crosses, and everyone had been astonished at the vigour and luxuriance of the new varieties which he has thus raised and afterwards fixed by selection. He gave me seed-peas produced from crosses between four distinct kinds; and the plants thus raised were extraordinarily vigorous, being in each case from 1 to 2 or even 3 feet taller than the parent-forms, which were raised at the same time close alongside. But as I did not measure their actual height I cannot give the exact ratio, but it must have been at least as 100 to 75. A similar trial was subsequently made with two other peas from a different cross, and the result was nearly the same. For instance, a crossed seedling between the Maple and Purple-podded pea was planted in poor soil and grew to the extraordinary height of 116 inches; whereas the tallest plant of either parent variety, namely, a Purple-podded pea, was only 70 inches in height; or as 100 to 60.

Sarothamnus scoparius.

Bees incessantly visit the flowers of the common Broom, and these are adapted by a curious mechanism for cross-fertilisation. When a bee alights on the wing-petals of a young flower, the keel is slightly opened and the short stamens spring out, which rub their pollen against the abdomen of the bee. If a rather older flower is visited for the first time (or if the bee exerts great force on a younger flower), the keel opens along its whole length, and the longer as well as the shorter stamens, together with the much elongated curved pistil, spring forth with violence. The flattened, spoon-like extremity of the pistil rests for a time on the back of the bee, and leaves on it the load of pollen with which it is charged. As soon as the bee flies away, the pistil instantly curls round, so that the stigmatic surface is now upturned and occupies a position, in which it would be rubbed against the abdomen of another bee visiting the same flower. Thus, when the pistil first escapes from the keel, the stigma is rubbed against the back of the bee, dusted with pollen from the longer stamens, either of the same or another flower; and afterwards against the lower surface of the bee dusted with pollen from the shorter stamens, which is often shed a day or two before that from the longer stamens. (5/16. These observations have been quoted in an abbreviated form by the Reverend G. Henslow, in the ‘Journal of Linnean Society Botany’ volume 9 1866 page 358. Hermann Muller has since published a full and excellent account of the flower in his ‘Befruchtung’ etc. page 240.) By this mechanism cross-fertilisation is rendered almost inevitable, and we shall immediately see that pollen from a distinct plant is more effective than that from the same flower. I need only add that, according to H. Muller, the flowers do not secrete nectar, and he thinks that bees insert their proboscides only in the hope of finding nectar; but they act in this manner so frequently and for so long a time that I cannot avoid the belief that they obtain something palatable within the flowers.

If the visits of bees are prevented, and if the flowers are not dashed by the wind against any object, the keel never opens, so that the stamens and pistil remain enclosed. Plants thus protected yield very few pods in comparison with those produced by neighbouring uncovered bushes, and sometimes none at all. I fertilised a few flowers on a plant growing almost in a state of nature with pollen from another plant close alongside, and the four crossed capsules contained on an average 9.2 seeds. This large number no doubt was due to the bush being covered up, and thus not exhausted by producing many pods; for fifty pods gathered from an adjoining plant, the flowers of which had been fertilised by the bees, contained an average of only 7.14 seeds. Ninety-three pods spontaneously self-fertilised on a large bush which had been covered up, but had been much agitated by the wind, contained an average of 2.93 seeds. Ten of the finest of these ninety-three capsules yielded an average of 4.30 seeds, that is less than half the average number in the four artificially crossed capsules. The ratio of 7.14 to 2.93, or as 100 to 41, is probably the fairest for the number of seeds per pod, yielded by naturally-crossed and spontaneously self-fertilised flowers. The crossed seeds compared with an equal number of the spontaneously self-fertilised seeds were heavier, in the ratio of 100 to 88. We thus see that besides the mechanical adaptations for cross-fertilisation, the flowers are much more productive with pollen from a distinct plant than with their own pollen.

Eight pairs of the above crossed and self-fertilised seeds, after they had germinated on sand, were planted (1867) on the opposite sides of two large pots. When several of the seedlings were an inch and a half in height, there was no marked difference between the two lots. But even at this early age the leaves of the self-fertilised seedlings were smaller and of not so bright a green as those of the crossed seedlings. The pots were kept in the greenhouse, and as the plants on the following spring (1868) looked unhealthy and had grown but little, they were plunged, still in their pots, into the open ground. The plants all suffered much from the sudden change, especially the self-fertilised, and two of the latter died. The remainder were measured, and I give the measurements in Table 5/58, because I have not seen in any other species so great a difference between the crossed and self-fertilised seedlings at so early an age.

TABLE 5/58. Sarothamnus scoparius (very young plants).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 4 4/8 : 2 4/8. Pot 1 : 6 : 1 4/8. Pot 1 : 2 : 1.

Pot 2 : 2 : 1 4/8. Pot 2 : 2 4/8 : 1. Pot 2 : 0 4/8 : 0 4/8.

Total : 17.5 : 8.0.

The six crossed plants here average 2.91, and the six self-fertilised 1.33 inches in height; so that the former were more than twice as high as the latter, or as 100 to 46.

In the spring of the succeeding year (1869) the three crossed plants in Pot 1 had all grown to nearly a foot in height, and they had smothered the three little self-fertilised plants so completely that two were dead; and the third, only an inch and a half in height, was dying. It should be remembered that these plants had been bedded out in their pots, so that they were subjected to very severe competition. This pot was now thrown away.

The six plants in Pot 2 were all alive. One of the self-fertilised was an inch and a quarter taller than any one of the crossed plants; but the other two self-fertilised plants were in a very poor condition. I therefore resolved to leave these plants to struggle together for some years. By the autumn of the same year (1869) the self-fertilised plant which had been victorious was now beaten. The measurements are shown in Table 5/59.

TABLE 5/59. Pot 2.—Sarothamnus scoparius.

Heights of plants measured in inches.

Column 1: Crossed Plants.

Column 2: Self-fertilised Plants.