OUTLINES OF BOTANY. xxiii 



204. Tubers, fleshy thickenings of the stem or other parts of the plant, succulent leaves or 

 branches, the fleshy, woody, or bony parts of fruits, the albumen, and the thick fleshy parts of 

 embryos, consist chiefly of largely developed cellular tissue, replete with starch or other 

 substances (192), deposited apparently in most cases for the eventual future use of the plant or 

 its parts when recalled into activity at the approach of a new season. 



205. Hairs (171) are usually expansions or processes of the epidermis, and consist of one or 

 more cells placed end to end. When thick or hardened into prickles, they still consist usually 

 of cellular tissue only. Thorns (170) contain more or less of a fibro-vascular system, according 

 to their degree of development. 



206. Glands, in the primary sense of the word (175, 1), consist usually of a rather loose cellular 

 tissue without epidermis, and often replete with resinous or other substances. 



§ 3. Growth of the Organs. 



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 form a 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. 



2D9. The roots of Exogens, 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 oommenoea the 

 following season. 



210. The Stem, including its 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 and 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 eases however the lower part of 

 a branch and more rarely (e.g. in some Meliacea:) the lower part of a compound leaf attains its 

 full size before the young leaves or leaflets of the extremity are yet formed. 



211. The perennial stem, if exogenous (198), grows in thickness by the addition every season 

 of a new layer or ring of wood between the outermost preceding layer and the inner surface ot 

 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 of wood, thus forming a succession 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 teipperate climates, the seasons of growth correspond with 

 the years, and the rings or wood remain sufficiently distinct to indicate the age of the tree ; but 

 in many tropical and some evergeen 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 therefore no certain criterion for judging of the age 

 of the 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 than 

 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 flo (vers in 

 this very early stage are supposed by some modern botanists to be more normal, that is, more in 



