(A1) archegonia, (A2) antheridia, and (A3) the rhizoids. B: Prothallus, showing the young plant with its first leaf (B1), its own roots (B3) and the rhizoids of the prothallus (B2). Drawing and legend for it slightly altered from Kraemer.
in the truest sense, merely a preparation for the process that will produce another fern, it is always known as a prothallus. The prothallus is thus the first stage in the reproduction of ferns, a very simple stage, with only the faintest indication that the thallus might be considered the vegetative and its rhizoids perhaps the rootlike counterparts of foliage and roots of mature ferns. As we shall see presently, even this differentiation has not the significance that such a structure in flowering plants would indicate. There is not, as yet, the faintest indication of sexes that need to mate in order to produce their young. The spore has so far only produced a tiny flat body of green tissues with a few rootlike threads, so unlike the fern from which it started that its true significance, or even the fact that it had ought to do with ferns was not known until about the middle of the last century.
This green cushiony prothallus keeps on growing, its heart-shaped mass becoming divided into an obviously left and right hand side and the rhizoids multiplying in number. They are always borne on the lower side next the ground, or next whatever the prothallus may be growing on. Near the notch of the heart-shaped prothallus are developed a few flask-shaped bodies which contain within them an egg cell or single ovum, the female reproductive body. By a series of changes this egg cell becomes embedded in a mucilaginous material. This flask-shaped body with the female egg cell inside is known as the archegonium. From among the rhizoids there may, at about the same time, be found developing small globular organs that have in them a number of tiny cells, each of which has attached many minute threadlike tails. The globular organs, with their minute, tailed cells are known as antheridia, and comprise the male reproductive equipment. Just as in flowering plants, neither the archegonia (female) nor the antheridia (male) can produce offspring without mating and the method by which this marriage is accomplished differs tremendously both in practice and in its significations from that in phanerogams. In the first place, the male and female reproductive cells are separated by a considerable distance, they are both inclosed in structurally different casings, and the whole operation is so microscopic that insects can be of no service. Nor can the wind do for them what we have seen that it does for the pollen of pines and grasses.
Of the aids to fertilization there remains then only the water, which plays such an important part in the mating of the eelgrass and ditch grass among flowering plants. But in these ferns a very different drama is about to be enacted. The male cells, as we have seen, are provided with slender tails, which are movable. They move, in fact, to such good purpose that the male cell can actually swim in the water. Of course its minute size demands only the merest drop of water, in which it will take the only excursion of its brief life. For just as soon as it is mature, a heavy dew or the tiniest particle of water will set free the little male messengers. The water too has not been without effect on the female cell. More remarkable still, this mucilaginous matter contains in it a substance that acts as a lure to the swimming male cells. In any event they do swim directly to the entrance of the female cell’s abode, through it and to her, when the union is effected. At once there is thrown across the entrance a membrane that excludes all other males, and the fertilization is complete. From this union of the male and female cells a true young fern begins to develop. First a young leaf and roots, finally a stem and in the end, of course, a full-grown fern producing spores, ready to renew the whole process.
Some ferns do not follow all the steps exactly as we have outlined, for all of them have not the structure of the typical one whose life history has been sketched above. In the adder’s-tongue fern, for instance there is a stalklike prolongation from the base of the only leaf the plant bears, on which all the spores are borne. In certain others, as in the ostrich fern, the spores are borne on leaflike growths that serve only this function. Most ferns, however, bear spores on otherwise unmodified foliage leaves and the great bulk of them on the under side of such leaves.
There are several things about the life history of a fern that differ fundamentally from any flowering plant and perhaps the chief is what is known as the alternation of generations. A spore, for instance, can never produce a fern as a seed will always produce a flowering plant. In this respect they are like many insects that always have two or sometimes three different stages in their life history. Only by the complicated method of first a spore then the prothallus, from which archegonia and antheridia are produced, followed by the free swimming male cells fertilizing the female, can a fern reproduce itself. As we shall see in the chapter on the History of the Plant Kingdom, this alternation of generations, the absolute necessity of water in which to carry on the fertilization, and above all the ability of the male cells for free swimming in the water, are all landmarks in the development of plant life. In its simplest form fertilization in flowerless plants is characterized by one or all these processes, as it is in the ferns, while in the flowering plants, the act is accomplished by processes, discussed previously, which, in the development of the plant kingdom, mark a period only comparable, in the history of man, to such tremendous achievements as the acquirement of speech or the ability to make a fire.
Ever since the war, the peat-forming mosses, known as sphagnum, have become more widely known to the general public than any of the ten or twelve thousand mosses known to grow on the earth. Its power of absorption, greater than linen bandages, made it extensively used to pad surgical dressings. Hundreds of thousands of these sphagnum
A Small Cloud of Wind-Blown Pollen of the Japanese Red Pine (Pinus densiflora). (Photo by C. Stuart Gager. Courtesy of Brooklyn Botanic Garden.)
A Coconut Grove in the Philippine Islands. The people of tropical regions have more uses for this plant than there are days of the year. Its fruits will float in the sea for months without injury and it is thought to have been distributed all over the tropical world by ocean currents. Its true wild home is not certainly known, but is probably tropical America. See chapter V for an account of the tree. (Courtesy of Brooklyn Botanic Garden.)
dressings were made, and the collection of sphagnum from the bogs in which it nearly always grows was the task of many who could render no other service.
The reproduction of sphagnum is not unlike that of ferns already described. There is the same necessity of a film of water in which the free swimming male can fertilize the female. But some other things about their reproduction of young differ from ferns.
In the first place sphagnum is a nonvascular cryptogam, in that its leaves have no veins or ducts in them and its minute stem is also without those conducting passages that characterize all ferns, and the flowering plants, which are considered the most highly developed of all plant life. (See Chapter I for a discussion of this point, in the section devoted to “Flowerless Plants.”)
In this moss, also, there are small branches, some of which bear only the tiny leaves, but some bear leaves and the reproductive organs. The female or archegonia are much like those in the ferns, and the antheridia or male are also, as in the ferns, minute globular organs in which are the male cells. The branches bearing males are greenish, yellow, or even reddish, quite unlike the ashy gray foliage leaves which give to sphagnum its characteristic ashy gray color. Unlike the ferns, the male cells of sphagnum have only two tails, but they nevertheless swim, tail first, to the female, when the time for fertilization comes. The female branches are found mostly toward the upper end of the plant and bear the archegonia at their extremities.
From what we know of the reproductive stages in the ferns it is now obvious enough that in sphagnum moss, as we ordinarily see it, we have, because it bears antheridia and archegonia, a quite different condition from the ordinary spore-bearing leaves of ferns. For as yet spores have not been developed on the moss. The mating of male and female cells, directly on the plant, proves that in this “plant,” at least, our ordinary notion of this moss is mostly confined to a stage in its life history comparable in ferns to the production of archegonia and antheridia on the fern prothallus. From this mating of the male and female cells there results, as in the ferns, the production of a spore-bearing structure. This consists of a spore case, matured for the most part in the chamber occupied by the fertilized female cell, but ultimately its cap is carried upward. Later on the spore case ruptures, releasing the spores. As in the ferns, these germinate, forming a short green protonema followed by a prothallus. From this a short leafy branch develops, which completes the life cycle, as this is the young moss plant.
In other words, sphagnum, as we ordinarily see it, produces, on the plant, male and female cells which unite to form a spore case with spores in it. These are shed, develop into a protonema which is followed by the prothallus and from this the young moss plant develops. In ferns the conspicuous well-known stage is the spore-bearing one, in sphagnum it is the production of male and female cells directly on what appears to be the mature plant.
There are many other kinds of mosses than sphagnum, and their life histories differ in slight degrees from it. But they all agree in this, that the greenish, feathery little moss plant is a stage in its life history bearing male and female cells, the mating of which produces a spore-bearing contrivance. In most of the familiar green mosses this is a capsulelike body on a short stalk, usually well elevated above the green mass of plants. From this the spores are shed and develop into a protonema or “first thread” just as in ferns. Unlike them, and unlike sphagnum, the green mosses produce no thallus, and the young leaves of the moss are developed directly from this protonema.
The common mushroom that we eat is easily enough divided into a thick stalk, known as a stipe, and a broad hood called a pileus. The under side of the pileus is seen to be composed of thin plaits set closely together and radiating from the center toward the edge. These are known as gills. From among the gills the spores are shed when they are mature, usually foretold by the changing of the color of the gills from whitish to purplish and even to brown or blackish. The spores are then shed and ready for the next stage. From what we already know about ferns and mosses, it is clear that from these spores a mushroom cannot develop without the production of male and female cells and all the rest of that process of hidden marriage that characterizes all flowerless plants. But in most mushrooms no one has ever seen, nor have the most carefully conducted experiments ever demonstrated the germination of the spore. So far as we know at the present, many mushrooms may or may not produce their young through the germination of their spores in their native fields and meadows and the subsequent production of male and female reproductive organs. But if their spores do produce such organs, which all our knowledge of spores makes probable, it is, in a truer sense than in most cryptogams, a case of hidden marriage. The process of producing their young is thus a secret one that scientists have not yet been able to disclose. Of course it is a common practice of mushroom growers to purchase spawn from seedsmen which under favorable conditions will produce many young mushroom plants. This, however, is the production of young without mating of the sexes, a fairly common characteristic of many other plants which will be considered presently.
As we saw in the section devoted to Flowerless Plants in Chapter I, there are many other kinds of fungi than the familiar edible mushroom and their close relatives, the often deadly poisonous toadstools. The reproductive processes in these other fungi are fairly well understood, but they can hardly be included here. In the mold on bread, the yeast used in baking, the rust of wheat and the diseases of other plants and of animals, the individual organism is so minute that it can only be detected under the microscope. Their reproductive processes are, of course, on such a minute scale that they could be followed with profit only by those equipped to study them. They have been described in many botanical textbooks, and those interested in them should consult such books.
In recapitulating the reproductive processes in cryptogamous plants the thing that distinguishes them from all flowering plants is that they bear, in some stage of their life history, a spore. From this, in the great bulk of them, a mature plant never develops. Only by the production from the spore of some contrivance for bearing male and female cells, which may, as in some seaweeds, even be on different plants, can a mating of these be accomplished, and from this union will develop the mature plant. There are many modifications of this plan, but in nearly all of them the presence of water, for the free swimming of the male cell to its mate, is essential. Just as in flowering plants and in all the larger animals, however, the reproduction of young in cryptogams is a sexual process depending on the union of male and female. While in phanerogams that process may well be spoken of as visible marriage, with all the pageantry of insects and beautifully colored flowers, in cryptogams the process is not only a hidden marriage, its ways are sometimes so secret that, even in the common mushroom, the actual mating is conjectured rather than demonstrated.
It is so generally true in all plants that a union of male and female is necessary for the production of young, and, as we have seen in most of them, the process is so uniformly successful that still another mode of producing them seems almost unnecessary. Yet in a surprisingly large number of plants new individuals, both of flowering and flowerless plants, are regularly produced without such a union and where sexuality has nothing to do with the increase.
In the life plant—a thick-leaved shrub from Mexico commonly grown in greenhouses—the leaves are wavy margined. From their edges, especially when injured, many tiny new plants will often start to grow. Even if the leaf is cut up into fairly small pieces many of these will develop young plants, and in various forms of the common rex begonia the leaves are usually cut into small pieces by gardeners for the production of young plants which always sprout from such pieces. It is useless to multiply such cases, as everyone knows of the production of young plants from the ends of strawberry runners, the cutting up of potatoes, the universal garden practice of making cuttings, and the sprouting of willows, all of which are effective by virtue of this faculty of plants to produce young quite without the intervention of different sexes. Not so well known are the cases of a liverwort, a small relative of the mosses, which, if chopped into fine pieces, each will develop into a new plant. We have already spoken of the spawn of mushrooms; and even on sphagnum moss, in addition to its sexual reproduction, it produces sterile branches that will root and, after separation from the old plant, form a new one.
Wherever this tendency is found, whether it be in a microscopic seaweed, some of which know no other means of reproduction, or in the showy begonia, it depends for its success upon a property of the ultimate unit of its structure, the cell. Sometimes, as in bacteria or the most minute seaweeds and in some other kinds, the whole plant consists of a single microscopic cell, when it is said to be a unicellular plant. All others, in which the grouping or modifications of the cell makes more complex structures, such as trees or shrubs and all the plants that grow, both flowering and flowerless, are called multicellular plants. Whether they be of one or many cells, these have the faculty of dividing, and by this division making two where one existed before the division. This division of cells is what happens in the normal growth of plants and it is this division, in more unusual ways, that results in the production of new plants without mating of the sexes. As cells are themselves microscopic, of course their division is equally so, and cannot be described in detail here. It has been many times described and pictured both in books on plants and animals, as it is the ultimate unit of the structure of both.
Plant life, then, seems to be better provided with means to renew itself than most animals, for, as we have seen, it has several methods to rely on. These may be divided into sexual, which includes both that in flowering plants with their visible mating and in flowerless plants with invisible mating, and asexual, literally without sex. In the latter are all those unicellular plants that reproduce themselves by simple division of the cell, and also those flowering plants that either naturally, as in life plant, or by the gardener’s art of making cuttings, produce new plants quite without the intervention of the sexes. Whether it be sexual or asexual, nature has more than fulfilled its obligation to the plant world in providing it opportunities for self-renewal. No matter what apparently unfavorable condition arises and often in spite of an almost unbelievable wastage of potential life stuff, the renewal goes on, or else there is the total disappearance of the species. So strong is this tendency to provide for renewal of their kind that many plants, if injured or cut by a mower, will almost in their last gasp hurriedly flower and set seeds, and we have already seen that the little liverwort, even if cut to pieces, also obeys that nearly universal law of nature: “Be fruitful and multiply.”
THERE are perhaps over 150,000 different kinds of flowering plants known in the world to-day, but the flowerless ones are fewer than these in numbers. No one really knows how many thousands of the cryptogams there may be in the world, for all of them have not yet been described, and there are doubtless thousands of which we merely suspect the existence. Flowering plants are so much better known, and have for 2,000 years been the subject of scientific writings, that their relationships and obvious groupings into families are fairly definite and often easily recognizable.
In our ordinary discussions or gossip of neighbors or relatives, the absolutely necessary starting point is to know their name. Their acquirement of this by christening, or by the adoption of it through the usage of parents, settles for life what they will be called. Plants are also christened, and that ceremony is one of the most important events in our subsequent discussion of them.
As we always have at least two names, one to show that we are a Smith, for instance, and another to fix us as John Smith, so all plants have two names, sometimes three.
And because plants come from all over the world and are studied and loved by people of many different languages, it became necessary very early in the descriptive writings about them to hit upon some device that should insure the name of a particular plant being the same all over the world, whether used by a student in the Imperial University of China, or by a garden enthusiast in Connecticut. At the time when this need for christening plants with names that would pass current throughout the world was getting to be a crying necessity, the language of all learned men was Latin, so it was natural that they should give Latin names to plants. That practice has continued to the present day, and there are even now some botanists that cling to the old custom of describing the newly christened plant in Latin. In the olden days this was always done, so that much of our knowledge of plants has come down to us from early books written wholly in Latin. The unfamiliarity of Latin to most of us, and the terrifyingly difficult spelling of some plant names, has resulted in many people saying: “God made the flowers, but the devil gave them names.” Nevertheless, these Latin names are the only ones we can use without endless confusion, just as we bear the names assigned to us by our parents, and no others.
If we were to go out into the country and pick up a wild rose which seemed to be different from any other rose, it would be necessary, in order to talk about it subsequently, to give it a name. After carefully searching through all the books about roses and finding out that it really is a new kind of rose rather than merely being new to us, we should then be ready to christen our new find. As we have said, all plants bear two names. One of these is a general one, like Smith, for instance, and the other more specific, like John. These general names of plants, and they are always their first names, are, because they fix the plant as belonging to a particular group, known as generic names. The generic name of violets, for instance, is Viola; of buttercups, Ranunculus; of wheat, Triticum; of corn, Zea; and of roses, Rosa. Our new rose then bears, without any act of ours, the generic name Rosa, which was applied to roses many years ago, and must therefore be used for all subsequently discovered roses. This generic name of Rosa, like all other generic names, tells us that roses are a well-recognized group of plants, all more like one another than like blackberries, for instance, and because of this they are said to all belong to the same genus. A genus (plural, genera) is a group of different plants, all more like one another than like anything else. To go back to our new Rosa, we must now apply its second and more specific name. If it were a white rose, and had never before been described, we should almost certainly use for its second name something signifying its color and assign alba as the obvious Latin equivalent of white. The second name is always called the specific name, because it shows us that from all other roses our new Rosa alba differs in being white. It is of the genus Rosa, but it is also and forever after a recognized member of Rosa, to which a specific name has been applied—in other words, Rosa alba is a species of rose. Species are thus plants more like one another than they are like any other member of the genus to which they belong. Rosa alba is a species quite unlike Rosa lucida, or Rosa carolina, or all the other scores of roses already known or described. At the time of christening Rosa alba, we should not only enter its name in a book, as ours would be in a parish register, but do much more than that. We should so carefully describe it and, preferably, illustrate it with a picture that no one coming after could ever mistake Rosa alba for any other rose. It can be readily seen that the christening of new plants is very nearly as serious an affair as christening babies, and furthermore, it is only to be attempted by experts. Because this has not always been done, many plants have been christened two or three times. Of course, these subsequent christenings do not seriously matter, for plants, like ourselves, should have only one specific name, the first applied to them. But their subsequent christenings by the careless and ignorant have enormously increased the difficulty of talking or writing about plants. These spurious names are common throughout the literature of botany and are known as synonyms.
The thing to remember about plants, so far as our need for classifying them is concerned, is that they belong to different species which might almost be considered the unit or simplest recognizable category into which they may be sorted. For convenience, we sort species into genera which may well be considered the next highest category in which plants are grouped. The grouping of genera into tribes, of tribes into families, and of families into still larger categories, has nothing to do with their names, but everything to do with our understanding of how they are related to one another, and what these different categories mean in the great collection of plants all about us. In other words, it reduces to a definite system an apparently hopelessly mixed-up mass of plants that, without some contrivance of the sort, would simply be a lot of totally unrelated specimens of plant life. Actually they are grouped in fairly definite categories, some of which are easily recognizable, and all of which fit into that great scheme of nature where everything may seem chaotic, but to the observant it is really a very pattern of order. What it all means and how plants have been grouped into families will be explained, now that we understand how they have acquired their generic and specific names.
A scientist once visiting in Bulgaria noticed that the peasants in that country frequently lived over a hundred years and, in trying to find out the reason, he discovered that they drank large quantities of sour milk. This is alive with a definite kind of bacterium that is of great benefit to the digestive apparatus, and therefore helps in the prolongation of life. In Bulgaria, in other words, a certain food habit of the people has resulted in a definite prolongation of life and fixes that population as of somewhat different characteristics from people not addicted to sour milk. In Japan a whole race lives largely on fish and rice, and while this is not the cause of their yellow skin, it is almost surely the cause of their generally small stature. Many of the English are tall, light-haired, and blue-eyed people, fond of outdoor life and sports, and among the most highly developed of the peoples of the earth. The climate of that island, their generally large consumption of meat and the outdoor life of so many of them, have resulted in quite definite characteristics that make the typical Englishman an easily distinguishable type.
In studying man we are able not only to divide him into such broad divisions as white, black, and yellow races, but due to their particular country or mode of life there are scores of racial subdivisions of these larger categories that everyone recognizes. Such differences are often based on stature, shape of head, mental characters and many others, but those still finer shades of difference between, for instance, a Connecticut Yankee and a plantation owner in the South, are, while noted by everyone, very difficult to accurately describe.
In attempting to find such major differences in plants, some structural character that would set off one large group of plants from every other group, the botanist has a harder task than the person studying man. For all those differences of language and mentality that make up such a large part of our common knowledge of the different peoples of the earth are characters that are foreign to plants. We are thus thrown back on structure as the chief way in which plants differ, and because their reproductive organs are their most important ones, and therefore least likely to vary, it is upon certain characters of these organs that all flowering plants have been divided.
In the chapter on “How Plants Produce Their Young,” we found that most flowering plants have their ovules in an ovary which, after fertilization, develop into fruit and seed. But some plants, while they have ovules, only bear them naked or between scales, never inclosed in an ovary. This is true in all pines, spruces, hemlocks, and all the host of their generally evergreen relatives. Such trees bear cones, between the scales of which are perfectly naked ovules that develop into seeds (Figure 77) that have never been hidden in an ovary, as have the vast majority of the seeds of other plants (Figure 53). These naked-seeded plants are known as gymnosperms or literally gymnos, naked, and sperma, seed, and comprise all the cone-bearing trees in the world, the larger part of which are always evergreen. In some past ages such trees made up the bulk of vegetation of the earth, but at present they are much reduced in numbers. Familiar examples of these Coniferæ, or cone-bearing trees, are larch, spruce, fir, pine, hemlock, juniper, and yew.
Most of these are evergreen, which does not, of course, mean that they bear the same leaves always, but that only a few drop off at a time and are so constantly renewed that the tree is actually ever green.
All other flowering plants always bear their ovules in an ovary and, because of this fact, are called angiosperms, literally angeion, a vessel, and sperma, seed. These inclosed seeded plants comprise the great bulk of the vegetation of the earth to-day. So far as the temperate zone is concerned, nearly all of them drop their leaves in the fall, and the trees belonging to the angiosperms are thus said to be deciduous trees.
No better idea of the present size and importance of these two groups of plants can be gained than to state the fact that perhaps not over 500 different kinds of gymnosperms, all of which are trees and shrubs, are known. All the rest of the flowering plants in the world, comprising over 150,000 different kinds of herbs, shrubs, and trees of infinite variety, are angiosperms and therefore bear ovules in an ovary, followed by seeds in or on some sort of a fruit. It would almost seem as though the simplest way to dispose of this great mass of plants would be to sort them into trees, shrubs, and herbs. For all of them belong to one of these types of plant growth, and the ancient students of plants, just before the time of Christ, actually divided all flowering plants into these three classes. This, of course, threw the coniferous trees in with all other kinds and, as we have already seen, they differ from all other kinds in the important character of having naked ovules.
Here, again, in order to get some system out of apparent chaos, we must fall back on some fundamental character. And, again, it is the product of the reproductive process in all this host of angiosperms which furnishes the clue. In the seeds of many of them the young embryo has folded up within it two seed leaves, while in all the rest only one. As we saw in Chapter I, these seeds germinate either with a single seed leaf, like corn (Figure 85), or with two seed leaves, like beans (Figure 81). Every one of these angiosperms belongs to one of these classes or the other, and perhaps more extraordinary still is the fact that in those with one seed leaf there are associated certain leaf and flower characters, while those with two seed leaves are always very different.
In the monocotyledons, or plants with a single seed leaf, the leaves are practically always parallel veined (Figure 83), like corn and grass, and lilies and palms, and hundreds of others. Also, they nearly always have the parts of their flowers in threes (Figure 84). That is, they have three sepals, petals, stamens, and often pistils, or multiples of three. The common trillium or wake-robin, for instance, has three sepals, three petals, six stamens, and three styles. With a few exceptions, and nature seems to delight in producing a few such, all monocotyledons have this parallel-veined leaf character and flower parts in threes or multiples of three.
Plants which send up two seed leaves (Figure 81), on the other hand, bear practically always netted-veined leaves (Figure 79), and the parts of their flowers are nearly always in fours or fives or multiples of these numbers (Figure 80). The well-known wild geranium has five sepals, five petals, ten stamens, and a five-lobed or five-celled ovary. There is some individual variation from this plan, sometimes one organ and sometimes another having more or less than the regular number. But so overwhelmingly true are these distinctions that dicotyledons, or plants with two seed leaves, and monocotyledons,
Dicotyledonous and Monocotyledonous growth habits contrasted. Figs. 78-81. The trunk of a dicotyledonous tree showing division of the wood into heartwood, sapwood, and cambium, which the removal of a piece of outer bark exposes. Note the net-veined leaf (79), the seedling with two seed leaves (81), and with the parts of the flower in 5’s (80). Figs. 82-85. Monocotyledonous plant. Note the lack of zones of wood, cambium and corky bark. Such plants have parallel-veined leaves (83), parts of their flowers in 3’s or 6’s (84), and germinate with a single seed leaf (85).
or plants with a single seed leaf, have been for hundreds of years the two great classes into which all angiospermous flowering plants are divided.
Our general view of all the flowering plants may be summarized then as follows:
1. Gymnosperms, or naked seeded plants, include all cone-bearing plants, mostly evergreen and always trees or shrubs. The pine is a familiar example.
2. Angiosperms, or inclosed seeded plants, include all other flowering plants of whatever kind. Divided into: (a) Monocotyledons. Sprouting with one seed leaf, and leaves practically always parallel-veined. Parts of the flower in threes or multiples of three. Familiar examples are corn, grass, sugar-cane, palms, cannas, and lily of the valley (Figures 82-85). (b) Dicotyledons. Sprouting with two seed leaves, and the leaves practically always netted-veined. Parts of the flower in fours or fives or multiples of these numbers. Includes all the remaining flowering plants and is a larger group than the monocotyledons and the cone-bearing plants combined (Figures 78-81).
No matter from what part of the world a totally unfamiliar plant may come, it is always possible to decide into which one of these groups it belongs. That in itself tells us a good deal about its ancestors and its future, “places” it, in fact, in one of those major groups into which all plants are divided. No other characters that plants possess are so important in determining their true position in the scale of plant life as those we have briefly outlined. But merely to sort plants into these large groups does not tell us all we need to know about them. For all plants not only belong to monocotyledons, or dicotyledons, or gymnosperms, but also to smaller divisions of these groups. Just as white men are divided into Englishmen, Frenchmen, etc., so there is the greatest necessity of dividing our large plant groups into smaller and more precise categories.
Some of the chief subdivisions of these large groups have been decided upon the fact that a considerable number of plants in them have some character in common, not found in the remaining plants of the group. Among the monocotyledons, for instance, there is a large class of plants that have tiny flowers between dry, chaffy scales, bear no true petals or sepals, all wind pollinated and are all commonly, though incorrectly, called grasses. These include, strictly speaking, two groups; one, the true grasses in which the stem is mostly hollow and the fruit a grain, while the other, with solid stems and bearing achenes for fruits, are the sedges. The grasses form one family and the sedges another, but while they differ in the characters just mentioned they agree in having flowers of the same general type. Families of plants are thus groups of genera, placed together in the scheme of classification, because they are more like one another than like any other such group. Among the grasses, for instance, are corn, wheat, rice, bamboo, orchard grass, Kentucky blue grass, sugar cane, and hundreds of others, all belonging to different genera, but all those genera grouped into a single family because of their generally similar flowers. Just as the Kentucky blue grass has a generic name (Poa) and a specific one (pratensis), the families of plants must also bear names, usually derived from the generic name of one of the chief genera in it. Because Poa is a large and important genus of the grasses, the family is named after it, with the addition of ceæ. Poaceæ is thus the family name of all the grasses. Among the sedges one of the commonest genera is Cyperus, including many species of the galingale or earth almond. From this genus the sedge family has been named Cyperaceæ (Figure 87). So the rose family is the Rosaceæ, the violet family is Violaceæ, and so on through all the three hundred or more families which contain all the flowering plants so far discovered. Going back for a moment to the Poaceæ and Cyperaceæ, the fact that these two large families are different from each other, but have some characters in common, fixes them as both belonging to one order. Orders are thus groups of one or more plant families, all differing one from another, but obviously related and having some characters in common. The order containing the grasses and sedges is named for one of the families in it with the ending ales. Thus Poales include Poaceæ and Cyperaceæ. Rosales include Rosaceæ and several families.
In other words, individual plants are grouped in species, species into genera, genera into families, and families into orders. These orders are themselves grouped into still larger divisions; there are, for instance, twelve orders comprising all the monocotyledons, and about twenty orders comprising all the dicotyledons. Once we have decided that any plant is a monocotyledon or a dicotyledon, our next step should be as to which order it belongs, then its family, its genus, and finally its species. Needless to say, such studies are necessarily of a technical nature, and while the details of them lie outside the scope of this book, the general plan or scheme of flowering plant classification is as we have outlined it above.
This scheme of plant classification has been developed not only for our convenience in sorting plants into definite categories, but more important still to show, if possible, the relationships, and particularly the development from the simplest to the most complex types of plant life. Thus the monocotyledons begin with the cat-tails, which have mere bristles for calyx and corolla, and lead by infinite gradations to the showy and highly complex orchids, which are considered the climax of the monocotyledonous families. While no general account of the plant families can be attempted here, some of the more interesting in both the monocotyledonous and dicotyledonous groups will be briefly discussed.
Of the simple plants of this group the Grass Family, or Poaceæ (Figure 86), is the most important, for in it are all our turf grasses, the bamboo and sugar cane, besides scores of others. Over 4,500 species are known, and they inhabit every region of the globe. The steppes of Russia and our Great Plains are predominately grassy; in the wonderful bamboo forests in the tropics are also woody representatives of this family. Certain kinds in the tropics grow as vines, with great hooked spines at the joints, so that nearly every kind of growth-form is to be found in the Poaceæ. All agree in having very small flowers, arranged in tiny spikelets, which are themselves grouped in various ways, although the inflorescence is usually some form of spike, or raceme or panicle. The individual flowers are between chaffy scales, of which several make up each spike. Always the lowest two scales are empty, and the flowers begin in the third from the bottom, or
Fig. 86. Blue-joint grass, a common grass of North America and a member of the Poaceæ. Fig. 87. Wool-grass, a tall swamp sedge popularly but incorrectly spoken of as grass. It is a member of the Cyperaceæ or Sedge family, which have usually triangular solid stems, whereas grasses have hollow round stems.
sometimes even above that. The flower is so simple that there is neither calyx nor corolla, only three stamens and one to three styles. The fruit is a grain and the Poaceæ, therefore, are the chief source of cereals. Wheat, rice, corn, oats, barley, millet, and rye, all come from grasses, and all, except corn, are natives of the Old World. They were grown for countless ages before the discovery of America, when Europeans first saw corn used by the Indians. As they are wind-pollinated, the flowers of grasses produce no honey nor colored petals, and the vast majority of them have no odor. Most of them reproduce, not only by seeds, but by rootstocks, and many of them grow so closely together that they form turf. In nearly all of them the stem is hollow, and in the largest of them, the bamboo, these hollow stems are used as water and sewer pipes, especially in India. An exception to the hollow stem is the sugar cane, from whose solid stem the juice is pressed out, that is the chief source of sugar; and our common Indian corn.
A plant of the lily family (Liliaceæ). Note the tendency to net-veined leaves in a monocotyledonous plant. Such instances are common in nature and net-veined leaves are found in certain species of smilax and most of the plants of the Arum family, containing the jack-in-the-pulpit, both monocotyledons.
Much more highly developed than the grasses is the lily family or Liliaceæ (Figure 88), but comprising less than 1,500 species in about 125 genera. They are nearly always herbs, but the Spanish bayonet forms a woody trunk, while the dragon tree of the Canary Islands is an extraordinary plant for a lily relative, one giant specimen of this being 80 feet tall and over 45 feet in circumference.[1] The flowers in the Liliaceæ are nearly always perfect, that is, stamens and pistils are found in the same flower. Its perianth segments are nearly always six, sometimes distinguishable as petals and sepals, but more often, as in the tulip, all colored similarly. The fruits are practically always a capsule that splits lengthwise. Perhaps the different plants in the Liliaceæ, as well as any others, illustrate the fact that plants of any particular family need not look like one another in order to be included in the same family. Nothing could be farther from resemblance than the bulb-bearing onion, the tulip, the Easter lily, the Spanish bayonet, and the dragon tree. Yet they and hundreds of other plants belong to the Liliaceæ. It cannot be overemphasized that it is flower and fruit characters that determine inclusion in any plant family, and similarity of leaves or habit may or may not accompany such characters. Among other well-known plants in the family, which is found throughout the world, are the crocus, the day lily, the dogtooth violet, hyacinth, and colchicum and aloes used in medicine. Many of them produce bulbs, such as onion, tulip, and lily and some of these contain valuable foods and drugs. The great majority of them are insect fertilized and are therefore wonderfully colored, and some furnish rich stores of honey.
But the most highly developed and interesting of all the monocotyledonous plants are the orchids (Figures 89-92). This family, Orchidaceæ,