LX VIII OUTLINES OF BOTANY. 
Thorns (170) contain more or less of a fibro-vascular system, according 
to egy degree of develo opment. 
ands, in the prim e of the word (175, 1), consist usually 
ey a gt ac loose cellular oe wionit epidermis, and often replete with 
resinous or other substan 
§ 3. Growth of the Organs. 
Roots grow in length constantly and regularly at the extremities 
only of their fibres, in proportion as they find the requisite nutriment. 
hey fo ur 8 i 
form n containing the germ of future branches, b 
fibres proceed irregularly from any part of their surface out previous 
indication, an wth has been stopped for a time, either 
ha 
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 circum- 
stances, be resu med at the same point, if the growing extrem ities | be 
t offt'd or stopped by a rock 
or other obstacle opposin g its progress, lateral fibres will be formed on 
the still living portion; thus en abling the root as a whole to diverge in 
any Sapa on, and travel far and wide when lured on by appropriate 
208 s growth is not however by the successive formation of hig 
minal ralle attaining at once their full size. _The cells first formed o 
fib 8 gr 
most Endogens the roots are annual, perishing at the close of the season, 
fresh adventitious roots s springing from the stock when vegetation com- 
mences the following season 
210. bie Stem, including - its branches and appendages (leaves, floral 
organs, etc.), grows in length by additions to its extremity, but a muc 
greater proportion of the extremity and branches remains in a growing 
and ye omnia state for a much longer time than in the case of the 
root. At the close 0 
= 
; > and un 
consist, ony lobe or tooth which is to mark ‘its margin, may ofte 
traced in miniature, and thenceforth, till it attains its full size, the sais 
grows and expands in every part. In some — however the lower 
part es a branch and more r arely (e. g. in some Meliaceae) the lower 
part compound leaf mgt its full size before the init leaves or 
ipatlote of the extremity are yet formed. 
211. The perennial stem, if exogenous (198), grows in thickness by med 
addition every season of a new layer or ring of wood between the o 
most preceding layer and ne inner surface of the bark, and by the 
