OUTLINES OF BOTANY. xvii 
proceed irregularly from any part of their surface without previous indica- 
tion, 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 particu- 
lar 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 en- 
abling 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 com- 
mencing or renewing its growth, will often dry up and form a kind of ter- 
minal 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 Jess 
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 eect Op 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 
ereater 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 commence- 
ment 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 Meliacee) 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 out- 
side 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 abso- 
lutely ascertained, into the cambiwm-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 suffi- 
ciently 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 
