The Elements of Botany Part 2

41. In Iris (Fig. 58, 59) the embryo in the seed is a small cylinder at one end of the ma.s.s of the alb.u.men, 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 c.h.i.n.k 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 alb.u.men, and transmitting it to the growing root below and new-forming leaves above.

[Ill.u.s.tration: Fig. 60. Section of an Onion seed showing the slender and coiled embryo in the alb.u.men, moderately magnified. 61. Seed of same in early germination.]

[Ill.u.s.tration: Fig. 62. Germinating Onion, more advanced, the c.h.i.n.k at base of cotyledon opening for the protrusion of the plumule, consisting of a thread-shaped leaf. 63. Section of base of Fig. 62, showing plumule enclosed. 64. Section of same later, plumule emerging. 65. Later stage of 62, upper part cut off. 66. A grain of Indian Corn, flatwise, cut away a little, so as to show the embryo, lying on the alb.u.men which makes the princ.i.p.al bulk of the seed. 67. A grain cut through the middle in the opposite direction, dividing the embryo through its thick cotyledon and its plumule, the latter consisting of two leaves, one enclosing the other. 68. The embryo taken out whole; the thick ma.s.s is the cotyledon, the narrow body partly enclosed by it is the plumule, the little projection at its base is the very short radicle enclosed in the sheathing base of the first leaf of the plumule.]

[Ill.u.s.tration: Fig. 69. Grain of Indian Corn in germination, the ascending sprout is the first leaf of the plumule, enclosing the younger leaves within, at its base the primary root has broken through. 70. The same, advanced; the second and third leaves developing, while the sheathing first leaf does not further develop.]

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 alb.u.men 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 c.h.i.n.k 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 cl.u.s.ter. 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 alb.u.men. 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 alb.u.men, but is not enclosed therein. The larger part of it is the cotyledon, thickish, its edges involute, and its back in contact with the alb.u.men; 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 alb.u.men, 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.

[Ill.u.s.tration: Fig. 71. Simple-stemmed vegetation.]

Section IV. GROWTH FROM BUDS: BRANCHING.

45. Most plants increase the amount of their vegetation by branching, that is, by producing lateral shoots.

46. Roots branch from any part and usually without definite order. Stems normally give rise to branches only at definite points, namely, at the nodes, and there only from the axils of leaves.

47. =Buds= (Fig. 72, 73). Every incipient shoot is a _Bud_ (12). A stem continues its growth by its _terminal bud_; it branches by the formation and development of _lateral buds_. As normal lateral buds occupy the axils of leaves, they are called _axillary buds_. As leaves are symmetrically arranged on the stem, the buds in their axils and the branches into which axillary buds grow partake of this symmetry. The most conspicuous buds are the scaly winter-buds of most shrubs and trees of temperate and cold climates; but the name belongs as well to the forming shoot or branch of any herb.

[Ill.u.s.tration: Fig. 72. Shoot of Horse-chestnut, of one year's growth, taken in autumn after the leaves have fallen; showing the large terminal bud and smaller axillary buds.]

[Ill.u.s.tration: Fig. 73. Similar shoot of s.h.a.gbark Hickory, Carya alba.]

48. =The Terminal Bud=, in the most general sense, may be said to exist in the embryo,--as cotyledons, or the cotyledons and plumule,--and to crown each successive growth of the simple stem so long as the summit is capable of growth. The whole ascending growth of the Palm, Cycas, and the like (such as in Fig. 71) is from a terminal bud. Branches, being repet.i.tions of the main stem and growing in the same way, are also lengthened by terminal buds. Those of Horse-chestnut, Hickory, Maples, and such trees, being the resting buds of winter, are conspicuous by their protective covering of scales. These bud-scales, as will hereafter be shown, are themselves a kind of leaves.

49. =Axillary Buds= were formed on these annual shoots early in the summer. Occasionally they grow the same season into branches; at least, some of them are pretty sure to do so whenever the growing terminal bud at the end of the shoot is injured or destroyed. Otherwise they may lie dormant until the following spring. In many trees or shrubs these axillary buds do not show themselves until spring; but if searched for, they may be detected, though of small size, hidden under the bark.

Sometimes, although early formed, they are concealed all summer long under the base of the leaf-stalk, which is then hollowed out into a sort of inverted cup, like a candle-extinguisher, to cover them; as in the Locust, the Yellow-wood, or more strikingly in the b.u.t.ton-wood or Plane-tree (Fig. 74).

[Ill.u.s.tration: Fig. 74. An axillary bud, concealed under the hollowed base of the leaf-stalk, in b.u.t.tonwood or Plane-tree.]

50. The _leaf-scars_, so conspicuous in Fig. 72, 73, under each axillary bud, mark the place where the stalk of the subtending leaf was attached until it fell in autumn.

51. =Scaly Buds=, which are well represented in Fig. 72, 73, commonly belong to trees and shrubs of countries in which growth is suspended during winter. The scaly coverings protect the tender young parts beneath, not so much by keeping out the cold, which of course would penetrate the bud in time, as by shielding the interior from the effects of sudden changes. There are all gradations between these and

52. =Naked Buds=, in which these scales are inconspicuous or wanting, as in most herbs, at least above ground, and most tropical trees and shrubs. But nearly related plants of the same climate may differ widely in this respect. Rhododendrons have strong and scaly winter-buds; while in Kalmia they are naked. One species of Viburnum, the Hobble-bush, has completely naked buds, what would be a pair of scales developing into the first leaves in spring; while another (the s...o...b..ll) has conspicuous scaly buds.

53. =Vigor of Vegetation from strong buds.= Large and strong buds, like those of the Horse-chestnut, Hickory, and the like, contain several leaves, or pairs of leaves, ready formed, folded and packed away in small compa.s.s, just as the seed-leaves of a strong embryo are packed away in the seed: they may even contain all the blossoms of the ensuing season, plainly visible as small buds. And the stems upon which these buds rest are filled with abundant nourishment, which was deposited the summer before in the wood or in the bark. Under the surface of the soil, or on it covered with the fallen leaves of autumn, similar strong buds of our perennial herbs may be found; while beneath are thick roots, rootstocks, or tubers, charged with a great store of nourishment for their use. This explains how it is that vegetation from such buds shoots forth so vigorously in the spring of the year, and clothes the bare and lately frozen surface of the soil, as well as the naked boughs of trees, very promptly with a covering of fresh green, and often with brilliant blossoms. Everything was prepared, and even formed, beforehand: the short joints of stem in the bud have only to lengthen, and to separate the leaves from each other so that they may unfold and grow. Only a small part of the vegetation of the season comes directly from the seed, and none of the earliest vernal vegetation. This is all from buds which have lived through the winter.

54. =The Arrangement of Branches=, being that of axillary buds, answers to that of the leaves. Now leaves princ.i.p.ally are either _opposite_ or _alternate_. Leaves are _opposite_ when there are two from the same joint of stem, as in Maples (Fig. 20), the two being on opposite sides of the stem; and so the axillary buds and branches are opposite, as in Fig. 75. Leaves are _alternate_ when there is only one from each joint of stem, as in the Oak, Lime-tree, Poplar, b.u.t.ton-wood (Fig. 74), Morning-Glory (Fig. 45,--not counting the seed-leaves, which of course are opposite, there being a pair of them); also in Indian Corn (Fig.

70), and Iris (Fig. 59). Consequently the axillary buds are also alternate, as in Hickory (Fig. 73); and the branches they form alternate,--making a different kind of spray from the other mode, one branch shooting on one side of the stem and the next on some other. For in the alternate arrangement no leaf is on the same side of the stem as the one next above or next below it.

55. But the symmetry of branches (unlike that of the leaves) is rarely complete. This is due to several causes, and most commonly to the

56. =Non-development of buds.= It never happens that all the buds grow.

If they did, there might be as many branches in any year as there were leaves the year before. And of those which do begin to grow, a large portion perish, sooner or later, for want of nourishment, or for want of light, or because those which first begin to grow have an advantage, which they are apt to keep, taking to themselves the nourishment of the stem, and starving the weaker buds. In the Horse-chestnut (Fig. 72), Hickory (Fig. 73), Magnolia, and most other trees with large scaly buds, the terminal bud is the strongest, and has the advantage in growth; and next in strength are the upper axillary buds: while the former continues the shoot of the last year, some of the latter give rise to branches, and the rest fail to grow. In the Lilac also (Fig. 75), the uppermost axillary buds are stronger than the lower; but the terminal bud rarely appears at all; in its place the uppermost pair of axillary buds grow, and so each stem branches every year into two,--making a repeatedly two-forked ramification, as in Fig. 76.

[Ill.u.s.tration: Fig. 75. Shoot of Lilac, with winter buds; the two uppermost axillary ones strong; the terminal not developed. 76. Forking ramification of Lilac; reduced in size.]

57. =Latent Buds.= Axillary buds that do not grow at the proper season, and especially those which make no appearance externally, may long remain latent, and at length upon a favorable occasion start into growth, so forming branches apparently out of place as they are out of time. The new shoots seen springing directly out of large stems may sometimes originate from such latent buds, which have preserved their life for years. But commonly these arise from

58. =Advent.i.tious Buds.= These are buds which certain shrubs and trees produce anywhere on the surface of the wood, especially where it has been injured. They give rise to the slender twigs which often feather the sides of great branches of our American Elms. They sometimes form on the root, which naturally is dest.i.tute of buds; they are even found upon some leaves; and they are sure to appear on the trunks and roots of Willows, Poplars, and Chestnuts, when these are wounded or mutilated.

Indeed Osier-Willows are _pollarded_, or cut off, from time to time, by the cultivator, for the purpose of producing a crop of slender advent.i.tious twigs, suitable for basket-work. Such branches, being altogether irregular, of course interfere with the natural symmetry of the tree. Another cause of irregularity, in certain trees and shrubs, is the formation of what are called

[Ill.u.s.tration: Fig. 77. Tartarean Honeysuckle, with three accessory buds in each axil.]

59. =Accessory or Supernumerary Buds.= There are cases where two, three, or more buds spring from the axil of a leaf, instead of the single one which is ordinarily found there. Sometimes they are placed one over the other, as in the Aristolochia or Pipe-Vine, and in the Tartarean Honeysuckle (Fig. 77); also in the Honey-Locust, and in the Walnut and b.u.t.ternut (Fig. 78), where the upper supernumerary bud is a good way out of the axil and above the others. And this is here stronger than the others, and grows into a branch which is considerably out of the axil, while the lower and smaller ones commonly do not grow at all. In other cases three buds stand side by side in the axil, as in the Hawthorn, and the Red Maple (Fig. 79.) If these were all to grow into branches, they would stifle each other. But some of them are commonly flower-buds: in the Red Maple, only the middle one is a leaf-bud, and it does not grow until after those on each side of it have expanded the blossoms they contain.

[Ill.u.s.tration: Fig. 78. b.u.t.ternut branch, with accessory buds, the uppermost above the axil.]

[Ill.u.s.tration: Fig. 79. Red-Maple branch, with accessory buds placed side by side. The annular lines toward the base in this and in Fig. 72 are scars of the bud-scales, and indicate the place of the winter-bud of the preceding year.]

60. =Sorts of Buds.= It may be useful to enumerate the kinds of buds which have been described or mentioned. They are

_Terminal_, when they occupy the summit of (or terminate) a stem,

_Lateral_, when they are borne on the side of a stem; of which the regular kind is the

_Axillary_, situated in the axil of a leaf. These are

_Accessory_ or _Supernumerary_, when they are in addition to the normal solitary bud; and these are _Collateral_, when side by side; _Superposed_, when one above another;

_Extra-axillary_, when they appear above the axil, as some do when superposed, and as occasionally is the case when single.

_Naked buds_; those which have no protecting scales.

_Scaly buds_; those which have protecting scales, which are altered leaves or bases of leaves.

_Leaf-buds_, contain or give rise to leaves, and develop into a leafy shoot.

_Flower-buds_, contain or consist of blossoms, and no leaves.

_Mixed buds_, contain both leaves and blossoms.

61. =Definite annual Growth= from winter buds is marked in most of the shoots from strong buds, such as those of the Horse-chestnut and Hickory (Fig. 72, 73). Such a bud generally contains, already formed in miniature, all or a great part of the leaves and joints of stem it is to produce, makes its whole growth in length in the course of a few weeks, or sometimes even in a few days, and then forms and ripens its buds for the next year's similar growth.

62. =Indefinite annual Growth=, on the other hand, is well marked in such trees or shrubs as the Honey-Locust, Sumac, and in sterile shoots of the Rose, Blackberry, and Raspberry. That is, these shoots are apt to grow all summer long, until stopped by the frosts of autumn or some other cause. Consequently they form and ripen no terminal bud protected by scales, and the upper axillary buds are produced so late in the season that they have no time to mature, nor has their wood time to solidify and ripen. Such stems therefore commonly die back from the top in winter, or at least all their upper buds are small and feeble; so the growth of the succeeding year takes place mainly from the lower axillary buds, which are more mature.

63. =Deliquescent and Excurrent Growth.= In the former case, and wherever axillary buds take the lead, there is, of course, no single main stem, continued year after year in a direct line, but the trunk is soon lost in the branches. Trees so formed commonly have rounded or spreading tops. Of such trees with _deliquescent_ stems,--that is, with the trunk dissolved, as it were, into the successively divided branches,--the common American Elm (Fig. 80) is a good ill.u.s.tration.

[Ill.u.s.tration: Fig. 80. An American Elm, with Spruce-trees, and on the left Arbor Vitae.]