The Garden Guide

Book: Gardening Science - the Vegetable Kingdom
Chapter: Chapter 6: Plant Physiology

Seed germination, plant growth and decay

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1055. The circle of vegetable life commences with the germination of a seed in the soil, and ends with the decay of the plant which is produced by that seed. The following correct and beauitiful picture of the progress of a plant from infancy to maturity, is given in Dr. Lindley's own words, and with his permission. 'If we place a seed,-that of an apple, for instance,-in earth at the temperature of 32ᆭ Fahr., it will remain inactive till it finally decays. But if it is placed in moist earth some degrees above 32ᆭ, and screened from the action of light, its integument gradually imbibes moisture and swells, the tissue is softened and acquires the capability of stretching, the water is decomposed, and a part of its oxygen combining with the carbon of the seed, forms carbonic acid, which is expelled; nutritious food for the young parts is prepared by the conversion of starch into sugar, and the vital action of the embryo commences. It lengthens downwards by the radicle, and upwards by the cotyledons; the former penetrating the soil, the latter elevating themselves above it, acquiring a green colour by the decomposition of the carbonic acid they absorb from the earth and atmosphere, and unfolding in the form of twe opposite roundish leaves. This is the first stage of vegetation: the young plant consists of little more than cellular tissue: only an imperfect developement of vascular and fibrous tissue being discoverable, in the form of a sort of cylinder lying just in the centre. The part within the cylinder, at its upper end, is now the pith, without it the bark; while the cylinder itself is the preparation for the medullary sheath, and consists of vertical tubes passing through, and separated by, cellular tissue. 'The young root is now lengthening at its point, and absorbing from the earth its nutriment, which passes up to the summit of the plant by the cellular substance, and is in part impelled into the cotyledons, where it is aerated and evaporated, but chiefly urged upwards against the growing point or plumule. 'The plumule, forced onwards by the current of sap. which is continually impelled upwards from the root, next ascends in the form of a little twig, at the same time sending downwards in the centre of the radicle, the earliest portion of wood that is deposited, and compelling the root to emit little remifications; and simultaneously the process of lignification is going on in all the tissue, by the deposit of a peculiar secretion in layers within the cells and tubes. Previously to the elongation of the plumule its point has acquired the rudimentary state of a leaf: this latter continues to develope as the plumule elongates, until, when the first internode of the latter ceases to lengthen, the leaf has actually arrived at its complete formation. When fully grown it repeats in a much more perfect manner the functions previously performed by the cotyledons: it aerates the sap that it receives, and returns the superfluous portion of it downwards through the bark to the root; tubular tissue at the same time appears between the medullary sheath and the bark, thus forming the first ligneous stratum, a part of which is incorporated with the bark, the remainder forming wood. During these operations, while the plumule is ascending, its leaf forming and acting, and the woody matter created by it descending, the cellular tissue of the item it forming, and expanding horizontally to make room for the new matter forced into it; so that developement is going on simultaneously both in a horizontal and perpendicular direction. This process may not inaptly be compared to that of weaving, the warp being the perpendicular, and the weft the horizontal, formation. In order to enable the leaf to perform its functions of aeration completely, it is traversed by veins originating in the medullary sheath, and has delicate pores (stomates), which communicate with a highly complex pneumatic system extending to almost every part of the plant. Simultaneously with the appearance of woody matter, the emission of young roots, and their increase by addition to the cellular substance of their points takes place. They thus are made to bear something like a definite proportion to the leaves they have to support, and with which they must of necessity be in direct communication. 'The plumule having produced its first leaf, others successively appear in a spiral direction around the axis at its growing point, all constructed alike, connected with the stem or axis in the same manner, and performing precisely the same functions as have been just described. At last the axis ceases to lengthen; the old leaves gradually fall off; the new leaves, instead of expanding after their formation, retain their rudimentary condition, harden, and fold over one another, so as to be a protection to the delicate point of growth; or, in other words, become the scales of a bud. We have now a shoot with a woody axis, and a distinct pith and bark ; and of a more or less conical figure. At the axil of every leaf a new growing point had been generated during the growth of the axis ; so that the shoot, when deprived of its leaves, is covered from end to end with little, symmetrically arranged, projecting bodies, which are the buds. The cause of the figure of the perfect shoot being conical is, that, as the wood originates in the base of the leaves, the lower end of the shoot, which has the greatest number of strata, because it has the greatest number of leaves above it, will be the thickest; and the upper end, which has had the fewest leaves to distend it by their deposit, will have the least diameter. Thus that part of the stem which has two leaves above it will have wood formed by two successive deposits; that which has nine leaves above it will have wood formed by nine successive deposits ; and so on : while the growing point, as it can have no deposit of matter from above, will have no wood, the extremity being merely covered by the rudiments of leaves hereafter to be developed. If at this time a cross section be examined, it will be found that the interior is no longer imperfectly divided into two portions, namely, pith and skin, as it was when first examined in the same way, but that it has distinctly two internal, perfect, concentric lines, the outer indicating a separation of the bark from the wood; and the inner, a separation of the wood from the pith: the latter too, which in the first observation was fleshy, and saturated with humidity, is become distinctly cellular, and altogether or nearly dry. 'Vegetation recommences with the spring of the second year and the return of warm weather. The uppermost, and perhaps some other buds, which were formed the previous year, gradually unfold, and pump up sap from the stock remaining in store about them ; the place of the sap so removed is instantly supplied by that which is next it; an impulse is thus given to the fluids from the summit to the roots; fresh extension and fresh fibrils are given to the roots; new sap is absorbed from the earth, and sent upwards through the wood of last year ; and the phenomenon called the flow of the sap is fully completed, to continue with greater or less velocity till the return of winter. The growing point lengthens upwards, forming leaves and buds in the same way as the parent shoot: the horizontal increase of the whole of the cellular system of the stem takes place, and each bud sends down organisable matter within the bark and above the wood of the shoot from which it sprang; thus forming on the one hand a new layer of wood, and on the other a fresh deposit of liber. In order to facilitate this last operation, the old bark and wood are separated in the spring by the exudation from both of them of the glutinous, slimy substance called cambium ; which appears to be expressly intended, in the first instance, to facilitate the developement of the subcortical tubular tissue; and, in the second place, to assist in generating the cellular tissue by which the horizontal dilatation of the axis is caused, and which maintains a communication between the bark and the centre of the stem. This communication has, by the second year, become sufficiently developed to be readily discovered, and is effected by the medullary rays. It will be remembered that there was a time when that which is now bark constituted a homogeneous body with the pith; and that it was after the leaves began to come into action that the separation which now exists between the bark and the pith took place. At the time when they were indissolubly united they both consisted of cellular tissue, with a few spiral vessels upon the line indicative of future separation. When a deposit of wood was formed from above between them they were not wholly divided the one from the other, but the deposit was effected in such a way as to leave a communication by means of cellular tissue between the bark and the pith; and, as this formation, or medullary ray, is at all times coetaneous with that of the wood, the communication so affected between the pith and bark is quite as perfect at the end of any number of years as it was at the beginning of the first; and so it continues to the end of the growth of the plant. The sap which is drawn from the earth into circulation by the unfolding leaves is exposed, as in the previous year, to the effect of air and light; is then returned through the petiole to the stem, and sent downwards through the bark, to be from it either conveyed to the root, or distributed horizontally by the medullary rays to the centre of the stem. At the end of the year the same phenomena occur as took place the first season: wood is gradually deposited by slower degrees, whence the last portion is denser than the first, and gives rise to the appearance called the annual zones: the new shoot or shoots are prepared for winter, and are again elongated cones ; and the original stem has acquired an increase in diameter proportioned to the quantity of new shoots which it produced, new shoots being to it now what young leaves were to it before. 'The third year. All that took place the year before is repeated; more roots appear; sap is again absorbed by the unfolding leaves; and its loss is made good by new fluids introduced by the roots and transmitted through the alburnum or wood of the year before; new wood and liber are formed from matter sent downwards by the buds : cambium is exuded; the horizontal developement of cellular tissue it repeated, but more extensively; wood towards the end of the year is formed more slowly, and has a more compact character; and another ring appears indicative of this year's increase. In precisely the same manner as in the second and third years of its existence will the plant continue to vegetate, till the period of its decay, each successive year being a repetition of the phenomena of that which preceded it. 'The age of puberty. After a certain number of years the tree arrives at the age of puberty: the period at which this occurs is very uncertain, depending in some measure upon adventitious circumstances, but more upon the idiosyncrasy, or peculiar constitution of the individual. About the time when this alteration of habit is induced, by the influence of which the sap or blood of the plant is to be partially diverted from its former courses into channels in which its force is to be applied to the production of new individuals rather than to the extension of itself; - about this time it will be remarked that certain of the young branches do not lengthen, as had been heretofore the wont of others, but assume a short stunted appearance, probably not growing two inches in the time which had been previously sufficient to produce twenty inches of increase. Of these little stunted branches, called spurs, the terminal bud acquires a swollen appearance, and at length, instead of giving birth to a new shoot, produces from its bosom a cluster of twigs in the form of pedicels, each terminated by a bud, the leaves of which are modified for the purposes of reproduction, grow firmly to each other, assume peculiar forms and colours, and form a flower, which had been enwrapped and protected from injury during the previous winter by several layers of imperfect leaves, now brought forth as bracts. Sap is impelled into the calyx through the pedicel by gentle degrees, is taken up by it, and exposed by the surface of its tube and segments to air and light; but having very imperfect means of returning, all that cannot be consumed by the calyx is forced onwards into the circulation of the petals, stamens, and pistil. The petals unfold themselves of a dazzling white tinged with pink, and expose the stamens; at the same time the disk changes into a saccharine substance, which is supposed to nourish the stamens and pistil, and give them energy to perform their functions. 'Propagation. At a fitting time, the stigmatic surface of the pistil being ready to receive the pollen, the latter is cast upon it from the anthers, which have remained near for that particular purpose. When the pollen touches the stigma, the grains adhere by means of its viscid surface, emitting a delicate membranous tube, which pierces into the stigmatic tissue, lengthens there, and conveys the matter contnined in the pollen towards the ovules, which the tube finally enters by means of their foramina. This has no sooner occurred than the petals and stamens fade and fall away, their ephemeral but important functions being accomplished. The sap which is afterwards impelled through the peduncle can only be disposed of to the calyx and ovary, where it lodges: both these swell and form a young fruit, which continues to grow as long as any new matter of growth is supplied from the parent plant. At this time the surface of the fruit performs the functions of leaves in exposing the juice to light and air ; at a subsequent period, it ceases to decompose carbonic acid, gains oxygen, loses its green colour, assumes the rich ruddy glow of maturity ; the peduncle, no longer a passage for fluids, dries up and becomes unequal to supporting the fruit, which at last falls to the earth. Here, if not destroyed by animals, it lies and decays: in the succeeding spring its seeds are stimulated into life, strike root, in the mass of decayed matter that surrounds them, and spring forth as new plants to undergo all the vicissitudes of their parent. 'Conclusion. Such are the progressive phenomena in the vegetation, not only of the apple, but of all trees that are natives of northern climates, and of a large part of the herbage of the same countries, - modified, of course, by peculiarities of structure and constitution, as in annual and herbaceous plants, and in those the leaves of which are opposite and not alternate; but all the more essential circumstances of their growth are the same as those of the apple tree. If we reflect upon these phenomena, our minds can scarcely fail to be deeply impressed with admiration at the perfect simplicity, and, at the same time, faultless skill with which all the machinery is contrived upon which vegetable life depends. A few forms of tissue, interwoven horizontally and perpendicularly, constitute a stem; the developement, by the first shoot that the seed produces, of buds which grow upon the same plan as the first shoot itself, and a constant repetition of the same formation causes an increase in the length and breadth of the plant; an expansion of the bark into a leaf, within which ramify veins proceeding from the seat of nutritive matter in the new shoot, with a provision of air-passages in its substance, and of pores on its surface, enables the crude fluid sent from the roots to be elaborated and digested until it becomes the peculiar secretion of the species; the contraction of a branch and its leaves forms a flower ; the disintegration of the internal tissue of a petal forms pollen; the folding inwards of a leaf is sufficient to constitute a pistil; and, finally, the gorging of the pistil with fluid which it cannot part with causes the production of a fruit. 'Endogenous, or monocotyledonous plants. In hot latitudes there exists another race of trees, of which palms are the representatives ; and in the north there are many herbs, in which growth, by addition to the outside, is wholly departed from, the reverse taking place ; that is to say, their diameter increasing by addition to the inside. As the seeds of such plants are formed with only one cotyledon, they are called monocotyledonous; and their growth being from the inside, they are also named endogenous. In these plants the functions of the leaves, flowers, and fruit are in nowise different from those of the apple; their peculiarity consisting only in the mode of forming their stems. When a monocotyledonous seed has vegetated, it usually does not disentangle its cotyledon from the testa, but simply protrudes the collum and the radicle; the cotyledon swelling, and remaining firmly encased in the seminal integuments. The radicle shoots downwards to become root; and a leaf is emitted from the side of the collum. This first leaf is succeeded by another facing it, and arising from its axil; the second produces a third half facing it, and arising also from its axil; and, in this manner, the spiral production of leaves continues, until the plant, if caulescent, is ready to produce its stem. Up to this period no stem having been formed, it has necessarily happened that the bases of the leaves hitherto produced have been all upon nearly the same plane; and as each has been produced from the bosom of the other without any such intervening space as occurs in dicotyledonous plants, it would be impossible for the matter of wood, if any were formed, to be sent downwards around the circumference of the plant; it would, on the contrary, have been necessarily deposited in the centre. In point of fact, however, no deposit of wood like that of dicotyledons takes place, either now or hereafter. The union of the bases of the leaves has formed a fleshy stock, cormus, or plate, which, if examined, will be found to consist of a mass of cellular tissue, traversed by perpendicular and horizontal bundles of vascular and woody tissue, connected with the veins of the leaves, of which they are manifest prolongations downwards; and there is no trace of separable bark, medullary rays, or central pith: the whole body being a mass of pith, woody, and vascular tissue, mixed together. To understand this formation yet more clearly, consider for a moment the internal structure of the petiole of a dicotyledon: it is composed of a bundle or bundles of vascular tissue encased in pleurenchym, surrounded on all sides with pith, or, which is the same thing, parenchym. Now suppose a number of these petioles to be separated from their blades, and to be tied in a bunch parallel with each other, and, by lateral pressure, to be squeezed so closely together that their surfaces touch each other accurately, except at the circumference of the bunch; if a transverse section of these be made, it will exhibit the same mixture of bundles of woody tissue and parenchym, and the same absence of distinction between pith, wood, and bark, which has been noticed in the corm, or first plate, of monocotyledons. 'Formation of the stems endegenous plants. As soon as the plate has arrived at the necessary diameter, it begins to lengthen upwards, leaving at its base those leaves that were before at its circumference, and carrying upwards with it such as occupied its centre; at the same time, new leaves continue to be generated at the centre, or, as it must now be called, at the apex of the shoot. As fresh leaves are developed, they thrust aside to the circumference those which preceded them, and a stem is by degrees produced. Since it has not been formed by additions made to its circumference by each successive leaf, it is not conical, as in dicotyledons; but, on the contrary, as its increase has been at the centre, which has no power to extend its limits, being confined by the circumference which, when once formed, does not afterwards materially alter in dimensions, it is, of necessity, cylindrical: and this is one of the marks by which a monocotyledon is often to be known in the absence of other evidence. The centre being but little acted upon by lateral pressure, remains loose in texture, and, until it becomes very old, does not vary much from the density acquired by it shortly after its formation; but the tissue of the circumference being continually jammed together by the pressure outwards of the new matter formed in the centre, in course of time becomes a solid mass of woody matter, the cellular tissue once intermingled with it being almost obliterated, and appearing among the bundles it formerly surrounded, like the interstices around the minute pebbles of a mosaic gem. 'Anomalous cases. Such is the mode of growth of palms, and of a great proportion of arborescent monocotyledons. But there are other monocotyledons in which this is in some measure departed from. In the common asparagus the shoots produce a number of lateral buds, which all develope and influence its form, as the buds of dicotyledons; so that the cylindrical figure of monocotyledons is exchanged for the conical; the internal structure remaining strictly endogenous. In grasses a similar conical figure prevails, and for the same reason; but they have this additional peculiarity, that their stem, in consequence of the great rapidity of its growth, is fistular, with transverse partitions at its nodes. The partitions are formed by the crossing of woody bundles from one side of the stem to the other; and are, perhaps, contrivances to enable the thin cylinder of the stem to resist pressure from without inwards. 'In such herbaceous plants as colchicum, the stem, after a time, is a small tuber with two buds; one at the apex, which becomes the flowering stem and leaves; the other at the base, directed downwards at an obtuse angle. Such a tuber is multiplied by the latter bud, which pushes forward obliquely, and, turning upwards, throws up a new flowering stem in the autumn; the base of the flowering stem thickens, enlarges, and assumes the appearance of a new corm; in the spring, leaves sprout forth, and elaborate matter enough to fill the cells of the new corm with starch, and to organise another oblique bud at the base; the growth of a new individual is then accomplished. In the meanwhile, the original corm is exhausted of all its organisable contents, which are consumed in the support of the young corm produced from its base: and, by the time that the growth of the latter is completed, the mother is shrivelled up, and dies. It is easy to conceive many modifications of this. 'Flowerless plants. Upon one or other of the two plans now explained are all flowering plants developed ; but in flowerless plants it is different. In arborescent ferns the stem consists of a cylinder of hard sinuous plates connected by parenchym, and surrounding an axis, hollow, or filled up with solid matter. It would seem, in these plants, as if the stem consisted of a mere adhesion of the petioles of the leaves in a single row; and that the stem simply lengthens at the point, without transmitting woody matter downwards. Some valuable observations upon this point have been made by Mohl, who has, however, been able only to investigate the anatomical condition of tree fern stems, without studying their mode of growth. Lycopods equally increase by simple addition to the point; and, as this seems also to be the plan upon which developement takes place in other cryptogamic plants, I have proposed the term Acrogens, to distinguish the latter from the classes of Exogens and Endogens. 'When leaves are no longer formed, but growth takes place by an irregular expansion of cellular tissue in various directions, the preceding rules are departed from, and nothing being left of the vegetable fabric except the horizontal order of growth, a stem ceases to appear, and a plant becomes an unsymmetrical body, either consisting of solid masses increasing in all directions, or of filamentous matter multiplying itself by internal septation at the elongated apex.' (Lindley's Introduction to Botany, 4th ed. (1848), vol. ii. p. 143.)