ELEMENTARY BOTANY. 
XXIX 
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), depositod 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. Thoms (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 unin- 
jured. 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 op- 
posing 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, consti- 
tuting a greater or lesser 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 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 commences 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 pro- 
duced 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 Meliacece) 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 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 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 
