OUTLINES OF BOTANY. 
XXIX 
usually is in communication with the ovary by a channel running down the centre of 
the style. 
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, re- 
plete 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 sub- 
stances. 
§ 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 contain- 
ing 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 nutri- 
ment at any particular spot, it will, on the return of favourable circumstances, be re- 
sumed 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 other- 
wise 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 attain- 
ing 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 
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 tor 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 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 succes- 
sion 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 
