§ 3. Growth of the Organs

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207. Roots grow in length constantly and regularly at the extremities only of their fibres, in proportion as they find the requisite nutriment. They form no buds containing the germ of future branches, but their fibres proceed irregularly from any part of their surface without previous indication, and when their growth has been stopped for a time, either wholly by the close of the season, or partially by a deficiency of nutriment at any particular spot, it will, on the return of favourable circumstances, be resumed at the same point, if the growing extremities be uninjured. If during the dead season, or at any other time, the growing extremity is cut off, dried up, or otherwise injured, or stopped by a rock or other obstacle opposing its progress, lateral fibres will be formed on the still living portion; thus enabling the root as a whole to diverge in any direction, and travel far and wide when lured on by appropriate nutriment.

208. This growth is not however by the successive formation of terminal cells attaining at once their full size. The cells first formed on a fibre commencing or renewing its growth, will often dry up and forma kind of terminal cap, which is pushed on as cells are formed immediately under it; and the new cells, constituting a greater or less portion of the ends of the fibres, remain some time in a growing state before they have attained their full size.

209. The roots of Exogons, when perennial, increase in thickness like stems by the addition of concentric layers, but these are usually much less distinctly marked; and in a large number of perennial Exogens and most Endogens the roots are annual, perishing at the close of the season, fresh adventitious roots springing from the stock when vegetation commences the following season.

210. The stem, including it branches and appendages (leaves, floral organs, etc.), grows in length by additions to its extremity, but a much greater proportion of the extremity and branches remains in a growing an expanding state for a much longer time than in the case of the root. At the close of one season, leaf-buds or seeds are formed, each containing the germ of a branch or young plant to be produced the following season. At a very early stage of the development of these buds or seeds, a commencement may be found of many of the leaves it is to bear; and before a leaf unfolds, every leaflet of which it is to consist, every lobe or tooth which is to mark its margin, may often be traced in miniature, and thenceforth, till it attains its full size, the branch grows and expands in every part. In some cases however the lower part of a branch and more rarely (e.g., in some Meliaceae) the lower part of a compound leaf attains its full size before the young leaves or leaflets at the extremity are yet formed.

211. The perennial stem, if exogenous (198), grows in thickness by the addition each season of a new layer or ring or wood between the outermost preceding layer and the inner surface of the bark, and by the formation of a new layer or ring of bark within the innermost preceding layer and outside the new ring of wood, thus forming a success of concentric circles. The sap elaborated by the leaves finds its way, in a manner not as yet absolutely ascertained, into the cambium-region, a zone of tender thin walled cells connecting the wood with the bark, by the division and enlargement of which new cells (190) are formed. These cells separate in layers, the inner ones constituting the new ring of wood, and the outer ones the new bark or liber. In most exogenous trees, in temperate climates, the seasons of growth correspond with the years, and the rings of wood remain sufficiently distinct to indicate the age of the tree; but in many tropical and some evergreen trees, two or more rings of wood are formed in one year.

212. In endogenous perennial stems (199), the new wood or woody fibre is formed towards the centre of the stem, or irregularly mingled with the old. The stem consequently either only becomes more dense without increasing in thickness, or only increases by gradual distension, which is never very considerable. It affords no certain criterion for judging of the age of the tree.

213. Flowers have generally all their parts formed, or indicated by protuberances or growing cells at a very early stage of the bud. These parts are then usually more regularly placed that in the fully developed flower. Parts which afterwards unite are then distinct, many are present in this rudimentary state which are never further developed, and parts which are afterwards very unequal or dissimilar are perfectly alike at this early period. On this account flowers in this very early stage are supposed by some modern botanists to be more normal, that is, more in conformity to a supposed type; and the study of the early formation and growth of the floral organs, called Organogenesis, has been considered essential for the correct appreciation of the affinities of plants. In some cases, however, it would appear that modifications of development, not to be detected in the very young bud, are yet of great importance in the distinction of large groups of plants, and that Organogenesis, although it may often assist in clearing up a doubtful point of affinity, cannot be relied on in estimating the real value of peculiarities of structure.

214. The flower is considered as a bud (flower bud, alabastrum) until the perianth expands, the period of flowering (anthesis) is that which elapses from the first expanding of the perianth until the pistil is set or begins to enlarge, or, when it does not set, until the stamens and pistils wither and fall. After that, the enlarged ovary takes the name of young fruit.

215. At the close of the season of growth, at the same time as the leaf-buds or seeds are formed containing the germ of future branches or plants, many plants form also, at or near the bud or seed, large deposits, chiefly of starch. In may cases — such as the tubers of a potato, or other root stock, the scales or thickened base of a bulb, the albumen or the thick cotyledons of a seed — this deposit appears to be a store of nutriment, which is partially absorbed by the young branch or plant during its first stage or growth, before the roots are sufficiently developed to supply it from without. In some cases, however, such as the fleshy thickening of some stems or peduncles, the pericarps of fruits which perish long before germination (the first growth of the seed), neither the use nor the cause of these deposits has as yet been clearly explained.

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