The Project Gutenberg eBook of The Effects of Cross & Self-fertilisation in the Vegetable Kingdom, by Charles Darwin

         28     : 33 2/8.
         27 3/8 : 23.
         27 5/8 : 21 5/8.
         28 6/8 : 20 4/8.
         29 7/8 : 21 5/8.
         26 6/8 : 22.
         26 2/8 : 21 2/8.
         30 1/8 : 21 7/8.

The average height of the crossed plants, whilst in full flower, was here 28.09, and that of the self-fertilised 23.14 inches; or as 100 to 82. It is a singular fact that the tallest plant in the two rows, was one of the self-fertilised. The self-fertilised plants had smaller and paler green leaves than the crossed. All the plants in the two rows were afterwards cut down and weighed. The twenty crossed plants weighed 65 ounces, and twenty self-fertilised (by calculation from the actual weight of the thirty-two self-fertilised plants) weighed 26.25 ounces; or as 100 to 40. Therefore the crossed plants did not exceed in weight the self-fertilised plants in nearly so great a degree as those growing in the pots, owing probably to the latter having been subjected to more severe mutual competition. On the other hand, they exceeded the self-fertilised in height in a slightly greater degree.

Plants of the common mignonette were raised from purchased seed, and several of them were placed under separate nets. Of these some became loaded with spontaneously self-fertilised capsules; others produced a few, and others not a single one. It must not be supposed that these latter plants produced no seed because their stigmas did not receive any pollen, for they were repeatedly fertilised with pollen from the same plant with no effect; but they were perfectly fertile with pollen from any other plant. Spontaneously self-fertilised seeds were saved from one of the highly self-fertile plants, and other seeds were collected from the plants growing outside the nets, which had been crossed by the bees. These seeds after germinating on sand were planted in pairs on the opposite sides of five pots. The plants were trained up sticks, and measured to the summits of their leafy stems—the flower-stems not being included. We here have the result:—

Heights of plants to the summits of the leafy stems, flower-stems not included, measured in inches.

Pot 1 : 20 7/8 : 22 4/8. Pot 1 : 34 7/8 : 28 5/8. Pot 1 : 26 6/8 : 23 2/8. Pot 1 : 32 6/8 : 30 4/8.

Pot 2 : 34 3/8 : 28 5/8. Pot 2 : 34 5/8 : 30 5/8. Pot 2 : 11 6/8 : 23. Pot 2 : 33 3/8 : 30 1/8.

Pot 3 : 17 7/8 : 4 4/8. Pot 3 : 27 : 25. Pot 3 : 30 1/8 : 26 3/8. Pot 3 : 30 2/8 : 25 1/8.

Pot 4 : 21 5/8 : 22 6/8. Pot 4 : 28 : 25 4/8. Pot 4 : 32 5/8 : 15 1/8. Pot 4 : 32 3/8 : 24 6/8.

The average height of the nineteen crossed plants is here 27.48, and that of the nineteen self-fertilised 22.55 inches; or as 100 to 82. All these plants were cut down in the early autumn and weighed: the crossed weighed 11.5 ounces, and the self-fertilised 7.75 ounces, or as 100 to 67. These two lots having been left freely exposed to the visits of insects, did not present any difference to the eye in the number of seed-capsules which they produced.

The remainder of the same two lots of seeds were sown in two adjoining rows in the open ground; so that the plants were exposed to only moderate competition. The eight tallest on each side were measured, as shown in Table 4/38.

         24 4/8 : 26 5/8.
         27 2/8 : 25 7/8.
         24     : 25.
         26 6/8 : 28 3/8.
         25     : 29 7/8.
         26 2/8 : 25 7/8.
         27 2/8 : 26 7/8.
         25 1/8 : 28 2/8.

The average height of the eight crossed plants is 25.76, and that of the eight self-fertilised 27.09; or as 100 to 105.

We here have the anomalous result of the self-fertilised plants being a little taller than the crossed; of which fact I can offer no explanation. It is of course possible, but not probable, that the labels may have been interchanged by accident.

Another experiment was now tried: all the self-fertilised capsules, though very few in number, were gathered from one of the semi-self-sterile plants under a net; and as several flowers on this same plant had been fertilised with pollen from a distinct individual, crossed seeds were thus obtained. I expected that the seedlings from this semi-self-sterile plant would have profited in a higher degree from a cross, than did the seedlings from the fully self-fertile plants. But my anticipation was quite wrong, for they profited in a less degree. An analogous result followed in the case of Eschscholtzia, in which the offspring of the plants of Brazilian parentage (which were partially self-sterile) did not profit more from a cross, than did the plants of the far more self-fertile English stock. The above two lots of crossed and self-fertilised seeds from the same plant of Reseda odorata, after germinating on sand, were planted on opposite sides of five pots, and measured as in the last case, with the result in Table 4/39.

Heights of plants to the summits of the leafy stems, flower-stems not included, measured in inches.

Pot 1 : 33 4/8 : 31. Pot 1 : 30 6/8 : 28. Pot 1 : 29 6/8 : 13 2/8. Pot 1 : 20 : 32.

Pot 2 : 22 : 21 6/8. Pot 2 : 33 4/8 : 26 6/8. Pot 2 : 31 2/8 : 25 2/8. Pot 2 : 32 4/8 : 30 4/8.

Pot 3 : 30 1/8 : 17 2/8. Pot 3 : 32 1/8 : 29 6/8. Pot 3 : 31 4/8 : 24 6/8. Pot 3 : 32 2/8 : 34 2/8.

Pot 4 : 19 1/8 : 20 6/8. Pot 4 : 30 1/8 : 32 6/8. Pot 4 : 24 3/8 : 31 4/8. Pot 4 : 30 6/8 : 36 6/8.

Pot 5 : 34 6/8 : 24 5/8. Pot 5 : 37 1/8 : 34. Pot 5 : 31 2/8 : 22 2/8. Pot 5 : 33 : 37 1/8.

The average height of the twenty crossed plants is here 29.98, and that of the twenty self-fertilised 27.71 inches; or as 100 to 92. These plants were then cut down and weighed; and the crossed in this case exceeded the self-fertilised in weight by a mere trifle, namely, in the ratio of 100 to 99. The two lots, left freely exposed to insects, seemed to be equally fertile.

The remainder of the seed was sown in two adjoining rows in the open ground; and the eight tallest plants in each row were measured, with the result in Table 4/40.

TABLE 4/40. Reseda odorata, (seedlings from a semi-self-sterile plant, planted in the open ground).

         28 2/8 : 22 3/8.
         22 4/8 : 24 3/8.
         25 7/8 : 23 4/8.
         25 3/8 : 21 4/8.
         29 4/8 : 22 5/8.
         27 1/8 : 27 3/8.
         22 4/8 : 27 3/8.
         26 2/8 : 19 2/8.

The average height of the eight crossed plants is here 25.92, and that of the eight self-fertilised plants 23.54 inches; or as 100 to 90.

Whilst the flowers of the common cultivated heartsease are young, the anthers shed their pollen into a little semi-cylindrical passage, formed by the basal portion of the lower petal, and surrounded by papillae. The pollen thus collected lies close beneath the stigma, but can seldom gain access into its cavity, except by the aid of insects, which pass their proboscides down this passage into the nectary. (4/5. The flowers of this plant have been fully described by Sprengel, Hildebrand, Delpino, and H. Muller. The latter author sums up all the previous observations in his ‘Befruchtung der Blumen’ and in ‘Nature’ November 20, 1873 page 44. See also Mr. A.W. Bennett in ‘Nature’ May 15, 1873 page 50 and some remarks by Mr. Kitchener ibid page 143. The facts which follow on the effects of covering up a plant of V. tricolor have been quoted by Sir J. Lubbock in his ‘British Wild Flowers’ etc. page 62.) Consequently when I covered up a large plant of a cultivated variety, it set only eighteen capsules, and most of these contained very few good seeds—several from only one to three; whereas an equally fine uncovered plant of the same variety, growing close by, produced 105 fine capsules. The few flowers which produce capsules when insects are excluded, are perhaps fertilised by the curling inwards of the petals as their wither, for by this means pollen-grains adhering to the papillae might be inserted into the cavity of the stigma. But it is more probable that their fertilisation is effected, as Mr. Bennett suggests, by Thrips and certain minute beetles which haunt the flowers, and which cannot be excluded by any net. Humble-bees are the usual fertilisers; but I have more than once seen flies (Rhingia rostrata) at work, with the under sides of their bodies, heads and legs dusted with pollen; and having marked the flowers which they visited, I found them after a few days fertilised. (4/6. I should add that this fly apparently did not suck the nectar, but was attracted by the papillae which surround the stigma. Hermann Muller also saw a small bee, an Andrena, which could not reach the nectar, repeatedly inserting its proboscis beneath the stigma, where the papillae are situated; so that these papillae must be in some way attractive to insects. A writer asserts ‘Zoologist’ volume 3-4 page 1225, that a moth (Plusia) frequently visits the flowers of the pansy. Hive-bees do not ordinarily visit them, but a case has been recorded ‘Gardeners’ Chronicle’ 1844 page 374, of these bees doing so. Hermann Muller has also seen the hive-bee at work, but only on the wild small-flowered form. He gives a list ‘Nature’ 1873 page 45, of all the insects which he has seen visiting both the large and small-flowered forms. From his account, I suspect that the flowers of plants in a state of nature are visited more frequently by insects than those of the cultivated varieties. He has seen several butterflies sucking the flowers of wild plants, and this I have never observed in gardens, though I have watched the flowers during many years.) It is curious for how long a time the flowers of the heartsease and of some other plants may be watched without an insect being seen to visit them. During the summer of 1841, I observed many times daily for more than a fortnight some large clumps of heartsease growing in my garden, before I saw a single humble-bee at work. During another summer I did the same, but at last saw some dark-coloured humble-bees visiting on three successive days almost every flower in several clumps; and almost all these flowers quickly withered and produced fine capsules. I presume that a certain state of the atmosphere is necessary for the secretion of nectar, and that as soon as this occurs the insects discover the fact by the odour emitted, and immediately frequent the flowers.

As the flowers require the aid of insects for their complete fertilisation, and as they are not visited by insects nearly so often as most other nectar-secreting flowers, we can understand the remarkable fact discovered by H. Muller and described by him in ‘Nature,’ namely, that this species exists under two forms. One of these bears conspicuous flowers, which, as we have seen, require the aid of insects, and are adapted to be cross-fertilised by them; whilst the other form has much smaller and less conspicuously coloured flowers, which are constructed on a slightly different plan, favouring self-fertilisation, and are thus adapted to ensure the propagation of the species. The self-fertile form, however, is occasionally visited, and may be crossed by insects, though this is rather doubtful.

In my first experiments on Viola tricolor I was unsuccessful in raising seedlings, and obtained only one full-grown crossed and self-fertilised plant. The former was 12 1/2 inches and the latter 8 inches in height. On the following year several flowers on a fresh plant were crossed with pollen from another plant, which was known to be a distinct seedling; and to this point it is important to attend. Several other flowers on the same plant were fertilised with their own pollen. The average number of seeds in the ten crossed capsules was 18.7, and in the twelve self-fertilised capsules 12.83; or as 100 to 69. These seeds, after germinating on bare sand, were planted in pairs on the opposite sides of five pots. They were first measured when about a third of their full size, and the crossed plants then averaged 3.87 inches, and the self-fertilised only 2.00 inches in height; or as 100 to 52. They were kept in the greenhouse, and did not grow vigorously. Whilst in flower they were again measured to the summits of their stems (see Table 4/41), with the following result:—

The average height of the fourteen crossed plants is here 5.58 inches, and that of the fourteen self-fertilised 2.37; or as 100 to 42. In four out of the five pots, a crossed plant flowered before any one of the self-fertilised; as likewise occurred with the pair raised during the previous year. These plants without being disturbed were now turned out of their pots and planted in the open ground, so as to form five separate clumps. Early in the following summer (1869) they flowered profusely, and being visited by humble-bees set many capsules, which were carefully collected from all the plants on both sides. The crossed plants produced 167 capsules, and the self-fertilised only 17; or as 100 to 10. So that the crossed plants were more than twice the height of the self-fertilised, generally flowered first, and produced ten times as many naturally fertilised capsules.

By the early part of the summer of 1870 the crossed plants in all the five clumps had grown and spread so much more than the self-fertilised, that any comparison between them was superfluous. The crossed plants were covered with a sheet of bloom, whilst only a single self-fertilised plant, which was much finer than any of its brethren, flowered. The crossed and self-fertilised plants had now grown all matted together on the respective sides of the superficial partitions still separating them; and in the clump which included the finest self-fertilised plant, I estimated that the surface covered by the crossed plants was about nine times as large as that covered by the self-fertilised plants. The extraordinary superiority of the crossed over the self-fertilised plants in all five clumps, was no doubt due to the crossed plants at first having had a decided advantage over the self-fertilised, and then robbing them more and more of their food during the succeeding seasons. But we should remember that the same result would follow in a state of nature even to a greater degree; for my plants grew in ground kept clear of weeds, so that the self-fertilised had to compete only with the crossed plants; whereas the whole surface of the ground is naturally covered with various kinds of plants, all of which have to struggle together for existence.

The ensuing winter was very severe, and in the following spring (1871) the plants were again examined. All the self-fertilised were now dead, with the exception of a single branch on one plant, which bore on its summit a minute rosette of leaves about as large as a pea. On the other hand, all the crossed plants without exception were growing vigorously. So that the self-fertilised plants, besides their inferiority in other respects, were more tender.

Another experiment was now tried for the sake of ascertaining how far the superiority of the crossed plants, or to speak more correctly, the inferiority of the self-fertilised plants, would be transmitted to their offspring. The one crossed and one self-fertilised plant, which were first raised, had been turned out of their pot and planted in the open ground. Both produced an abundance of very fine capsules, from which fact we may safely conclude that they had been cross-fertilised by insects. Seeds from both, after germinating on sand, were planted in pairs on the opposite sides of three pots. The naturally crossed seedlings derived from the crossed plants flowered in all three pots before the naturally crossed seedlings derived from the self-fertilised plants. When both lots were in full flower, the two tallest plants on each side of each pot were measured, and the result is shown in Table 4/42.

TABLE 4/42. Viola tricolor: seedlings from crossed and self-fertilised plants, the parents of both sets having been left to be naturally fertilised.

The average height of the six tallest plants derived from the crossed plants is 12.56 inches; and that of the six tallest plants derived from the self-fertilised plants is 10.31 inches; or as 100 to 82. We here see a considerable difference in height between the two sets, though very far from equalling that in the previous trials between the offspring from crossed and self-fertilised flowers. This difference must be attributed to the latter set of plants having inherited a weak constitution from their parents, the offspring of self-fertilised flowers; notwithstanding that the parents themselves had been freely intercrossed with other plants by the aid of insects.

The results of my experiments on this plant are hardly worth giving, as I remark in my notes made at the time, “seedlings, from some unknown cause, all miserably unhealthy.” Nor did they ever become healthy; yet I feel bound to give the present case, as it is opposed to the general results at which I have arrived. Fifteen flowers were crossed and all produced fruit, containing on an average 32.5 seeds; nineteen flowers were fertilised with their own pollen, and they likewise all yielded fruit, containing a rather larger average of 34.5 seeds; or as 100 to 106. Seedlings were raised from these seeds. In one of the pots all the self-fertilised plants died whilst quite young; in the two others, the measurements were as follows:

The average height of the four crossed plants is 14.25, and that of the four self-fertilised plants 14.31; or as 100 to 100.4; so that they were in fact of equal height. According to Professor H. Hoffman, this plant is proterandrous (4/7. ‘Zur Speciesfrage’ 1875 page 11.); nevertheless it yields plenty of seeds when protected from insects.

It has been said in the case of this plant, as of so many others, that the flowers are fertilised in the bud, and that distinct plants or varieties can never naturally intercross. (4/8. Decaisne ‘Comptes-Rendus’ July 1863 page 5.) But this is an error, as we may infer, firstly from the flowers being proterandrous,—the mature stamens bending up, one after the other, into the passage which leads to the nectary, and afterwards the mature pistils bending in the same direction; secondly, from the number of humble-bees which visit the flowers (4/9. Their structure is described by H. Muller ‘Befruchtung’ etc., page 122.); and thirdly, from the greater fertility of the flowers when crossed with pollen from a distinct plant than when spontaneously self-fertilised. In the year 1863 I enclosed a large branch in a net, and crossed five flowers with pollen from a distinct plant; these yielded capsules containing on an average 35.2 very fine seeds, with a maximum of forty-two in one capsule. Thirty-two other flowers on the same branch produced twenty-eight spontaneously self-fertilised capsules, containing on an average 17.2 seeds, with a maximum in one of thirty-six seeds. But six of these capsules were very poor, yielding only from one to five seeds; if these are excluded, the remaining twenty-two capsules give an average of 20.9 seeds, though many of these seeds were small. The fairest ratio, therefore, for the number of seeds produced by a cross and by spontaneous self-fertilisation is as 100 to 59. These seeds were not sown, as I had too many other experiments in progress.

In the summer of 1867, which was a very unfavourable one, I again crossed several flowers under a net with pollen from a distinct plant, and fertilised other flowers on the same plant with their own pollen. The former yielded a much larger proportion of capsules than the latter; and many of the seeds in the self-fertilised capsules, though numerous, were so poor that an equal number of seeds from the crossed and self-fertilised capsules were in weight as 100 to 45. The two lots were allowed to germinate on sand, and pairs were planted on the opposite sides of four pots. When nearly two-thirds grown they were measured, as shown in Table 4/44.

The six crossed plants here average 14.95, and the six self-fertilised 12.50 inches in height; or as 100 to 84. When fully grown they were again measured, but from want of time only a single plant on each side was measured; so that I have thought it best to give the earlier measurements. At the later period the three tallest crossed plants still exceeded considerably in height the three tallest self-fertilised, but not in quite so great a degree as before. The pots were left uncovered in the greenhouse, but whether the flowers were intercrossed by bees or self-fertilised I do not know. The six crossed plants produced 282 mature and immature capsules, whilst the six self-fertilised plants produced only 159; or as 100 to 56. So that the crossed plants were very much more productive than the self-fertilised.

Twelve flowers were crossed with pollen from another plant, and yielded ten capsules, containing by weight 5.77 grains of seeds. Eighteen flowers were fertilised with their own pollen and yielded twelve capsules, containing by weight 2.63 grains. Therefore the seeds from an equal number of crossed and self-fertilised flowers would have been in weight as 100 to 38. I had previously selected a medium-sized capsule from each lot, and counted the seeds in both; the crossed one contained 284, and the self-fertilised one 126 seeds; or as 100 to 44. These seeds were sown on opposite sides of three pots, and several seedlings raised; but only the tallest flower-stem of one plant on each side was measured. The three on the crossed side averaged 32.5 inches, and the three on the self-fertilised side 34 inches in height; or as 100 to 104. But this trial was on much too small a scale to be trusted; the plants also grew so unequally that one of the three flower-stems on the crossed plants was very nearly twice as tall as that on one of the others; and one of the three flower-stems on the self-fertilised plants exceeded in an equal degree one of the others.

In the following year the experiment was repeated on a larger scale: ten flowers were crossed on a new set of plants and yielded ten capsules containing by weight 6.54 grains of seed. Eighteen spontaneously self-fertilised capsules were gathered, of which two contained no seed; the other sixteen contained by weight 6.07 grains of seed. Therefore the weight of seed from an equal number of crossed and spontaneously self-fertilised flowers (instead of artificially fertilised as in the previous case) was as 100 to 58.

The seeds after germinating on sand were planted in pairs on the opposite sides of four pots, with all the remaining seeds sown crowded in the opposite sides of a fifth pot; in this latter pot only the tallest plant on each side was measured. Until the seedlings had grown about 5 inches in height no difference could be perceived in the two lots. Both lots flowered at nearly the same time. When they had almost done flowering, the tallest flower-stem on each plant was measured, as shown in Table 4/45.

The fifteen crossed plants here average 34.5, and the fifteen self-fertilised 33.55 inches in height; or as 100 to 97. So that the excess of height of the crossed plants is quite insignificant. In productiveness, however, the difference was much more plainly marked. All the capsules were gathered from both lots of plants (except from the crowded and unproductive ones in Pot 5), and at the close of the season the few remaining flowers were added in. The fourteen crossed plants produced 381, whilst the fourteen self-fertilised plants produced only 293 capsules and flowers; or as 100 to 77.

The common carnation is strongly proterandrous, and therefore depends to a large extent upon insects for fertilisation. I have seen only humble-bees visiting the flowers, but I dare say other insects likewise do so. It is notorious that if pure seed is desired, the greatest care is necessary to prevent the varieties which grow in the same garden from intercrossing. (4/10. ‘Gardeners’ Chronicle’ 1847 page 268.) The pollen is generally shed and lost before the two stigmas in the same flower diverge and are ready to be fertilised. I was therefore often forced to use for self-fertilisation pollen from the same plant instead of from the same flower. But on two occasions, when I attended to this point, I was not able to detect any marked difference in the number of seeds produced by these two forms of self-fertilisation.

Several single-flowered carnations were planted in good soil, and were all covered with a net. Eight flowers were crossed with pollen from a distinct plant and yielded six capsules, containing on an average 88.6 seeds, with a maximum in one of 112 seeds. Eight other flowers were self-fertilised in the manner above described, and yielded seven capsules containing on an average 82 seeds, with a maximum in one of 112 seeds. So that there was very little difference in the number of seeds produced by cross-fertilisation and self-fertilisation, namely, as 100 to 92. As these plants were covered by a net, they produced spontaneously only a few capsules containing any seeds, and these few may perhaps be attributed to the action of Thrips and other minute insects which haunt the flowers. A large majority of the spontaneously self-fertilised capsules produced by several plants contained no seeds, or only a single one. Excluding these latter capsules, I counted the seeds in eighteen of the finest ones, and these contained on an average 18 seeds. One of the plants was spontaneously self-fertile in a higher degree than any of the others. On another occasion a single covered-up plant produced spontaneously eighteen capsules, but only two of these contained any seed, namely 10 and 15.

CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.

The many seeds obtained from the above crossed and artificially self-fertilised flowers were sown out of doors, and two large beds of seedlings, closely adjoining one another, thus raised. This was the first plant on which I experimented, and I had not then formed any regular scheme of operation. When the two lots were in full flower, I measured roughly a large number of plants but record only that the crossed were on an average fully 4 inches taller than the self-fertilised. Judging from subsequent measurements, we may assume that the crossed plants were about 28 inches, and the self-fertilised about 24 inches in height; and this will give us a ratio of 100 to 86. Out of a large number of plants, four of the crossed ones flowered before any one of the self-fertilised plants.

Thirty flowers on these crossed plants of the first generation were again crossed with pollen from a distinct plant of the same lot, and yielded twenty-nine capsules, containing on an average 55.62 seeds, with a maximum in one of 110 seeds.

Thirty flowers on the self-fertilised plants were again self-fertilised; eight of them with pollen from the same flower, and the remainder with pollen from another flower on the same plant; and these produced twenty-two capsules, containing on an average 35.95 seeds, with a maximum in one of sixty-one seeds. We thus see, judging by the number of seeds per capsule, that the crossed plants again crossed were more productive than the self-fertilised again self-fertilised, in the ratio of 100 to 65. Both the crossed and self-fertilised plants, from having grown much crowded in the two beds, produced less fine capsules and fewer seeds than did their parents.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

The crossed and self-fertilised seeds from the crossed and self-fertilised plants of the last generation were sown on opposite sides of two pots; but the seedlings were not thinned enough, so that both lots grew very irregularly, and most of the self-fertilised plants after a time died from being smothered. My measurements were, therefore, very incomplete. From the first the crossed seedlings appeared the finest, and when they were on an average, by estimation, 5 inches high, the self-fertilised plants were only 4 inches. In both pots the crossed plants flowered first. The two tallest flower-stems on the crossed plants in the two pots were 17 and 16 1/2 inches in height; and the two tallest flower-stems on the self-fertilised plants 10 1/2 and 9 inches; so that their heights were as 100 to 58. But this ratio, deduced from only two pairs, obviously is not in the least trustworthy, and would not have been given had it not been otherwise supported. I state in my notes that the crossed plants were very much more luxuriant than their opponents, and seemed to be twice as bulky. This latter estimate may be believed from the ascertained weights of the two lots in the next generation. Some flowers on these crossed plants were again crossed with pollen from another plant of the same lot, and some flowers on the self-fertilised plants again self-fertilised; and from the seeds thus obtained the plants of the next generation were raised.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

The seeds just alluded to were allowed to germinate on bare sand, and were planted in pairs on the opposite sides of four pots. When the seedlings were in full flower, the tallest stem on each plant was measured to the base of the calyx. The measurements are given in Table 4/46. In Pot 1 the crossed and self-fertilised plants flowered at the same time; but in the other three pots the crossed flowered first. These latter plants also continued flowering much later in the autumn than the self-fertilised.

The average height of the eight crossed plants is here 28.39 inches, and of the eight self-fertilised 28.21; or as 100 to 99. So that there was no difference in height worth speaking of; but in general vigour and luxuriance there was an astonishing difference, as shown by their weights. After the seed-capsules had been gathered, the eight crossed and the eight self-fertilised plants were cut down and weighed; the former weighed 43 ounces, and the latter only 21 ounces; or as 100 to 49.

These plants were all kept under a net, so that the capsules which they produced must have been all spontaneously self-fertilised. The eight crossed plants produced twenty-one such capsules, of which only twelve contained any seed, averaging 8.5 per capsule. On the other hand, the eight self-fertilised plants produced no less than thirty-six capsules, of which I examined twenty-five, and, with the exception of three, all contained seeds, averaging 10.63 seeds per capsule. Thus the proportional number of seeds per capsule produced by the plants of crossed origin to those produced by the plants of self-fertilised origin (both lots being spontaneously self-fertilised) was as 100 to 125. This anomalous result is probably due to some of the self-fertilised plants having varied so as to mature their pollen and stigmas more nearly at the same time than is proper to the species; and we have already seen that some plants in the first experiment differed from the others in being slightly more self-fertile.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

Twenty flowers on the self-fertilised plants of the last or third generation, in Table 4/46, were fertilised with their own pollen, but taken from other flowers on the same plants. These produced fifteen capsules, which contained (omitting two with only three and six seeds) on an average 47.23 seeds, with a maximum of seventy in one. The self-fertilised capsules from the self-fertilised plants of the first generation yielded the much lower average of 35.95 seeds; but as these latter plants grew extremely crowded, nothing can be inferred with respect to this difference in their self-fertility. The seedlings raised from the above seeds constitute the plants of the fourth self-fertilised generation in Table 4/47.

Twelve flowers on the same plants of the third self-fertilised generation, in Table 4/46, were crossed with pollen from the crossed plants in the same table. These crossed plants had been intercrossed for the three previous generations; and many of them, no doubt, were more or less closely inter-related, but not so closely as in some of the experiments with other species; for several carnation plants had been raised and crossed in the earlier generations. They were not related, or only in a distant degree, to the self-fertilised plants. The parents of both the self-fertilised and crossed plants had been subjected to as nearly as possible the same conditions during the three previous generations. The above twelve flowers produced ten capsules, containing on an average 48.66 seeds, with a maximum in one of seventy-two seeds. The plants raised from these seeds may be called the INTERCROSSED.

Lastly, twelve flowers on the same self-fertilised plants of the third generation were crossed with pollen from plants which had been raised from seeds purchased in London. It is almost certain that the plants which produced these seeds had grown under very different conditions to those to which my self-fertilised and crossed plants had been subjected; and they were in no degree related. The above twelve flowers thus crossed all produced capsules, but these contained the low average of 37.41 seeds per capsule, with a maximum in one of sixty-four seeds. It is surprising that this cross with a fresh stock did not give a much higher average number of seeds; for, as we shall immediately see, the plants raised from these seeds, which may be called the LONDON-CROSSED, benefited greatly by the cross, both in growth and fertility.

The above three lots of seeds were allowed to germinate on bare sand. Many of the London-crossed germinated before the others, and were rejected; and many of the intercrossed later than those of the other two lots. The seeds after thus germinating were planted in ten pots, made tripartite by superficial divisions; but when only two kinds of seeds germinated at the same time, they were planted on the opposite sides of other pots; and this is indicated by blank spaces in one of the three columns in Table 4/47. A 0 in the table signifies that the seedling died before it was measured; and a + signifies that the plant did not produce a flower-stem, and therefore was not measured. It deserves notice that no less than eight out of the eighteen self-fertilised plants either died or did not flower; whereas only three out of the eighteen intercrossed, and four out of the twenty London-crossed plants, were in this predicament. The self-fertilised plants had a decidedly less vigorous appearance than the plants of the other two lots, their leaves being smaller and narrower. In only one pot did a self-fertilised plant flower before one of the two kinds of crossed plants, between which there was no marked difference in the period of flowering. The plants were measured to the base of the calyx, after they had completed their growth, late in the autumn.

The average height of the sixteen London-crossed plants in Table 4/47 is 32.82 inches; that of the fifteen intercrossed plants, 28 inches; and that of the ten self-fertilised plants, 26.55.

These three lots of plants, which it should be remembered were all derived on the mother-side from plants of the third self-fertilised generation, fertilised in three different ways, were left exposed to the visits of insects, and their flowers were freely crossed by them. As the capsules of each lot became ripe they were gathered and kept separate, the empty or bad ones being thrown away. But towards the middle of October, when the capsules could no longer ripen, all were gathered and were counted, whether good or bad. The capsules were then crushed, and the seed cleaned by sieves and weighed. For the sake of uniformity the results are given from calculation, as if there had been twenty plants in each lot.

The sixteen London-crossed plants actually produced 286 capsules; therefore twenty such plants would have produced 357.5 capsules; and from the actual weight of the seeds, the twenty plants would have yielded 462 grains weight of seeds.

The fifteen intercrossed plants actually produced 157 capsules; therefore twenty of them would have produced 209.3 capsules and the seeds would have weighed 208.48 grains.

The ten self-fertilised plants actually produced 70 capsules, therefore twenty of them would have produced 140 capsules; and the seeds would have weighed 153.2 grains.

NUMBER OF CAPSULES:

WEIGHT OF SEEDS PRODUCED BY AN EQUAL NUMBER OF PLANTS OF THE THREE LOTS.

WEIGHT OF SEED:

We thus see how greatly the offspring from the self-fertilised plants of the third generation crossed by a fresh stock, had their fertility increased, whether tested by the number of capsules produced or by the weight of the contained seeds; this latter being the more trustworthy method. Even the offspring from the self-fertilised plants crossed by one of the crossed plants of the same stock, notwithstanding that both lots had been long subjected to the same conditions, had their fertility considerably increased, as tested by the same two methods.

In conclusion it may be well to repeat in reference to the fertility of these three lots of plants, that their flowers were left freely exposed to the visits of insects and were undoubtedly crossed by them, as may be inferred from the large number of good capsules produced. These plants were all the offspring of the same mother-plants, and the strongly marked difference in their fertility must be attributed to the nature of the pollen employed in fertilising their parents; and the difference in the nature of the pollen must be attributed to the different treatment to which the pollen-bearing parents had been subjected during several previous generations.

COLOUR OF THE FLOWERS.

The flowers produced by the self-fertilised plants of the last or fourth generation were as uniform in tint as those of a wild species, being of a pale pink or rose colour. Analogous cases with Mimulus and Ipomoea, after several generations of self-fertilisation, have been already given. The flowers of the intercrossed plants of the fourth generation were likewise nearly uniform in colour. On the other hand, the flowers of the London-crossed plants, or those raised from a cross with the fresh stock which bore dark crimson flowers, varied extremely in colour, as might have been expected, and as is the general rule with seedling carnations. It deserves notice that only two or three of the London-crossed plants produced dark crimson flowers like those of their fathers, and only a very few of a pale pink like those of their mothers. The great majority had their petals longitudinally and variously striped with the two colours,—the groundwork tint being, however, in some cases darker than that of the mother-plants.

Many flowers on this Hibiscus were crossed with pollen from a distinct plant, and many others were self-fertilised. A rather larger proportional number of the crossed than of the self-fertilised flowers yielded capsules, and the crossed capsules contained rather more seeds. The self-fertilised seeds were a little heavier than an equal number of the crossed seeds, but they germinated badly, and I raised only four plants of each lot. In three out of the four pots, the crossed plants flowered first.

The four crossed plants average 13.25, and the four self-fertilised 14.43 inches in height; or as 100 to 109. Here we have the unusual case of self-fertilised plants exceeding the crossed in height; but only four pairs were measured, and these did not grow well or equally. I did not compare the fertility of the two lots.