Section Iii. Morphology of Seedlings.

18. Having obtained a general idea of the growth and parts of a phanerogamous plant from the common Flax of the field, the seeds and seedlings of other familiar plants may be taken up, and their variations from the assumed pattern examined.

19. =Germinating Maples= are excellent to begin with, the parts being so much larger than in Flax that a common magnifying glass, although convenient, is hardly necessary. The only disadvantage is that fresh seeds are not readily to be had at all seasons.

20. The seeds of Sugar Maple ripen at the end of summer, and germinate in early spring. The embryo fills the whole seed, in which it is nicely packed; and the nature of the parts is obvious even before growth begins. There is a stemlet (caulicle) and a pair of long and narrow seed-leaves (cotyledons), doubled up and coiled, green even in the seed, and in germination at once unfolding into the first pair of foliage-leaves, though of shape quite unlike those that follow.

21. Red Maple seeds are ripe and ready to germinate at the beginning of summer, and are therefore more convenient for study. The cotyledons are crumpled in the seed, and not easy to straighten out until they unfold themselves in germination. The story of their development into the seedling is told by the accompanying Fig. 14-20; and that of Sugar Maple is closely similar. No plumule or bud appears in the embryo of these two Maples until the seed-leaves have nearly attained their full growth and are acting as foliage-leaves, and until a root is formed below. There is no great store of nourishment in these thin cotyledons; so further growth has to wait until the root and seed-leaves have collected and elaborated sufficient material for the formation of the second internode and its pair of leaves, which lending their help the third pair is more promptly produced, and so on.

22. Some change in the plan comes with the Silver or Soft White Maple. (Fig. 21-25). This blossoms in earliest spring, and it drops its large and ripened keys only a few weeks later. Its cotyledons have not at all the appearance of leaves; they are short and broad, and (as there is no room to be saved by folding) they are straight, except a small fold at the top,--a vestige of the habit of Maples in general. Their unusual thickness is due to the large store of nutritive matter they contain, and this prevents their developing into actual leaves. Correspondingly, their caulicle does not lengthen to elevate them above the surface of the soil; the growth below the cotyledons is nearly all of root. It is the little plumule or bud between them which makes the upward growth, and which, being well fed by the cotyledons, rapidly develops the next pair of leaves and raises them upon a long internode, and so on. The cotyledons all the while remain below, in the husk of the fruit and seed, and perish when they have yielded up the store of food which they contained.

23. So, even in plants so much alike as Maples, there is considerable difference in the amount of food stored up in the cotyledons by which the growth is to be made; and there are corresponding differences in the germination. The larger the supply to draw upon, the stronger the growth, and the quicker the formation of root below and of stem and leaves above. This deposit of food thickens the cotyledons, and renders them less and less leaf-like in proportion to its amount.

24. =Examples of Embryos with thickened Cotyledons.= In the Pumpkin and Squash (Fig. 26, 27), the cotyledons are well supplied with nourishing matter, as their sweet taste demonstrates. Still, they are flat and not very thick. In germination this store is promptly utilized in the development of the caulicle to twenty or thirty times its length in the seed, and to corresponding thickness, in the formation of a cluster of roots at its lower end, and the early production of the incipient plumule; also in their own growth into efficient green leaves. The case of our common Bean (Phaseolus vulgaris, Fig. 28-30) is nearly the same, except that the cotyledons are much more gorged; so that, although carried up into the air and light upon the lengthening caulicle, and there acquiring a green color, they never expand into useful leaves. Instead of this, they nourish into rapid growth the plumule, which is plainly visible in the seed, as a pair of incipient leaves; and these form the first actual foliage.

25. Very similar is the germination of the Beech (Fig. 31-33), except that the caulicle lengthens less, hardly raising the cotyledons out of the ground. Nothing would be gained by elevating them, as they never grow out into efficient leaves; but the joint of stem belonging to the plumule lengthens well, carrying up its pair of real foliage-leaves.

26. It is nearly the same in the Bean of the Old World (Vicia Faba, here called Horse Bean and Windsor Bean): the caulicle lengthens very little, does not undertake to elevate the heavy seed, which is left below or upon the surface of the soil, the flat but thick cotyledons remaining in it, and supplying food for the growth of the root below and the plumule above. In its near relative, the Pea (Fig. 34, 35), this use of cotyledons for storage only is most completely carried out. For they are thickened to the utmost, even into hemispheres; the caulicle does not lengthen at all; merely sends out roots from the lower end, and develops its strong plumule from the upper, the seed remaining unmoved underground. That is, in technical language, the germination is hypogaeous.

27. There is sufficient nourishment in the cotyledons of a pea to make a very considerable growth before any actual foliage is required. So it is the stem-portion of the plumule which is at first conspicuous and strong-growing. Here, as seen in Fig. 35, its lower nodes bear each a useless leaf-scale instead of an efficient leaf, and only the later ones bear leaves fitted for foliage.

28. This hypogaeous germination is exemplified on a larger scale by the Oak (Fig. 36, 37) and Horse-chestnut (Fig. 38, 39); but in these the downward growth is wholly a stout tap-root. It is not the caulicle; for this lengthens hardly any. Indeed, the earliest growth which carries the very short caulicle out of the shell comes from the formation of foot-stalks to the cotyledons; above these develops the strong plumule, below grows the stout root. The growth is at first entirely, for a long time mainly, at the expense of the great store of food in the cotyledons. These, after serving their purpose, decay and fall away.

29. Such thick cotyledons never separate; indeed, they sometimes grow together by some part of their contiguous faces; so that the germination seems to proceed from a solid bulb-like mass. This is the case in a horse-chestnut.

30. =Germinating Embryo supplied by its own Store of Nourishment=, i. e. the store in the cotyledons. This is so in all the illustrations thus far, essentially so even in the Flax. This nourishment was supplied by the mother plant to the ovule and seed, and thence taken into the embryo during its growth. Such embryos, filling the whole seed, are comparatively large and strong, and vigorous in germination in proportion to the amount of their growth while connected with the parent plant.

31. =Germinating Embryo supplied from a Deposit outside of Itself.= This is as common as the other mode; and it occurs in all degrees. Some seeds have very little of this deposit, but a comparatively large embryo, with its parts more or less developed and recognizable. In others this deposit forms the main bulk of the seed, and the embryo is small or minute, and comparatively rudimentary. The following illustrations exemplify these various grades. When an embryo in a seed is thus surrounded by a white substance, it was natural to liken the latter to the white of an egg, and the embryo or germ to the yolk. So the matter around or by the side of the embryo was called the Albumen, i. e. the white of the seed. The analogy is not very good; and to avoid ambiguity some botanists call it the ENDOSPERM. As that means in English merely the inwards of a seed, the new name is little better than the old one; and, since we do not change names in botany except when it cannot be avoided, this name of albumen is generally kept up. A seed with such a deposit is albuminous, one with none is exalbuminous.

32. The ALBUMEN forms the main bulk of the seed in wheat, maize, rice, buckwheat, and the like. It is the floury part of the seed. Also of the cocoa-nut, of coffee (where it is dense and hard), etc.; while in peas, beans, almonds, and in most edible nuts, the store of food, although essentially the same in nature and in use, is in the embryo itself, and therefore is not counted as anything to be separately named. In both forms this concentrated food for the germinating plant is food also for man and for animals.

33. For an albuminous seed with a well-developed embryo, the common Morning Glory (Ipomoea purpurea, Fig. 40-43) is a convenient example, being easy and prompt to grow, and having all the parts well apparent. The seeds (duly soaked for examination) and the germination should be compared with those of Sugar and Red Maple (19-21). The only essential difference is that here the embryo is surrounded by and crumpled up in the albumen. This substance, which is pulpy or mucilaginous in fresh and young seeds, hardens as the seed ripens, but becomes again pulpy in germination; and, as it liquefies, the thin cotyledons absorb it by their whole surface. It supplements the nutritive matter contained in the embryo. Both together form no large store, but sufficient for establishing the seedling, with tiny root, stem, and pair of leaves for initiating its independent growth; which in due time proceeds as in Fig. 44, 45.

34. Smaller embryos, less developed in the seed, are more dependent upon the extraneous supply of food. The figures 46-53 illustrate four grades in this respect. The smallest, that of the Peony, is still large enough to be seen with a hand magnifying glass, and even its cotyledons may be discerned by the aid of a simple stage microscope.

35. The broad cotyledons of Mirabilis, or Four-o'clock (Fig. 52, 53), with the slender caulicle almost encircle and enclose the floury albumen, instead of being enclosed in it, as in the other illustrations. Evidently here the germinating embryo is principally fed by one of the leaf-like cotyledons, the other being out of contact with the supply. In the embryo of Abronia (Fig. 54, 55), a near relative of Mirabilis, there is a singular modification; one cotyledon is almost wanting, being reduced to a rudiment, leaving it for the other to do the work. This leads to the question of the

36. =Number of Cotyledons.= In all the preceding illustrations, the embryo, however different in shape and degree of development, is evidently constructed upon one and the same plan, namely, that of two leaves on a caulicle or initial stem,--a plan which is obvious even when one cotyledon becomes very much smaller than the other, as in the rare instance of Abronia (Fig. 54, 55). In other words, the embryos so far examined are all

37. =Dicotyledonous=, that is, two-cotyledoned. Plants which are thus similar in the plan of the embryo agree likewise in the general structure of their stems, leaves, and blossoms; and thus form a class, named from their embryo DICOTYLEDONES, or in English, DICOTYLEDONOUS PLANTS. So long a name being inconvenient, it may be shortened into DICOTYLS.

38. =Polycotyledonous= is a name employed for the less usual case in which there are more than two cotyledons. The Pine is the most familiar case. This occurs in all Pines, the number of cotyledons varying from three to twelve; in Fig. 56, 57 they are six. Note that they are all on the same level, that is, belong to the same node, so as to form a circle or whorl at the summit of the caulicle. When there are only three cotyledons, they divide the space equally, are one third of the circle apart. When only two they are 180 deg. apart, that is, are opposite.

39. The case of three or more cotyledons, which is constant in Pines and in some of their relatives (but not in all of them), is occasional among Dicotyls. And the polycotyledonous is only a variation of the dicotyledonous type,--a difference in the number of leaves in the whorl; for a pair is a whorl reduced to two members. Some suppose that there are really only two cotyledons even in a Pine embryo, but these divided or split up congenitally so as to imitate a greater number. But as leaves are often in whorls on ordinary stems, they may be so at the very beginning.

40. =Monocotyledonous= (meaning with single cotyledon) is the name of the one-cotyledoned sort of embryo. This goes along with peculiarities in stem, leaves, and flowers, which all together associate such plants into a great class, called MONOCOTYLEDONOUS PLANTS, or, for shortness, MONOCOTYLS. It means merely that the leaves are alternate from the very first.

41. In Iris (Fig. 58, 59) the embryo in the seed is a small cylinder at one end of the mass of the albumen, with no apparent distinction of parts. The end which almost touches the seed coat is caulicle, the other end belongs to the solitary cotyledon. In germination the whole lengthens (but mainly the cotyledon) only enough to push the proximate end fairly out of the seed; from this end the root is formed, and from a little higher the plumule later emerges. It would appear therefore that the cotyledon answers to a minute leaf rolled up, and that a chink through which the plumule grows out is a part of the inrolled edges. The embryo of Indian Corn shows these parts on a larger scale and in a more open state (Fig. 66-68). There, in the seed, the cotyledon remains, imbibing nourishment from the softened albumen, and transmitting it to the growing root below and new-forming leaves above.

42. The general plan is the same in the Onion (Fig. 60-65), but with a striking difference. The embryo is long, and coiled in the albumen of the seed. To ordinary examination it shows no distinction of parts. But germination plainly shows that all except the lower end of it is cotyledon. For after it has lengthened into a long thread, the chink from which the plumule in time emerges is seen at the base, or near it, so the caulicle is extremely short, and does not elongate, but sends out from its base a simple root, and afterwards others in a cluster. Not only does the cotyledon lengthen enormously in the seedling, but (unlike that of Iris, Indian Corn, and all the cereal grains) it raises the comparatively light seed into the air, the tip still remaining in the seed and feeding upon the albumen. When this food is exhausted and the seedling is well established in the soil, the upper end decays and the emptied husk of the seed falls away.

43. In Maize or Indian Corn (Fig. 66-70), the embryo is more developed in the seed, and its parts can be made out. It lies against the starchy albumen, but is not enclosed therein. The larger part of it is the cotyledon, thickish, its edges involute, and its back in contact with the albumen; partly enclosed by it is the well-developed plumule or bud which is to grow. For the cotyledon remains in the seed to fulfil its office of imbibing nourishment from the softened albumen, which it conveys to the growing sprout; the part of this sprout which is visible is the first leaf of the plumule rolled up into a sheath and enclosing the rudiments of the succeeding leaves, at the base enclosing even the minute caulicle. In germination the first leaf of the plumule develops only as a sort of sheath, protecting the tender parts within; the second and the third form the first foliage. The caulicle never lengthens: the first root, which is formed at its lower end, or from any part of it, has to break through the enclosing sheath; and succeeding roots soon spring from all or any of the nodes of the plumule.

44. =Simple-stemmed Plants= are thus built up, by the continuous production of one leaf-bearing portion of stem from the summit of the preceding one, beginning with the initial stem (or caulicle) in the embryo. Some Dicotyls and many Monocotyls develop only in this single line of growth (as to parts above ground) until the flowering state is approached. For some examples, see Cycas (Fig. 71, front, at the left); a tall Yucca or Spanish Bayonet, and two Cocoa-nut Palms behind; at the right, a group of Sugar-canes, and a Banana behind.