THE TREE—ITS ROOT-SYSTEM 91 
that the substances for making new roots and shoots, 
new wood, and new leaves, &c., are constructed. It isin 
the increased area of this leaf laboratory that the larger 
supplies of salts, dissolved in the larger quantities of 
water from the soil, are brought into relations with the 
increased quantities of carbonaceous substance obtained 
from the air in the chlorophyll corpuscles, and conse- 
quently a larger yield of plant-forming materials is pos- 
sible to meet the demands of the ever growing organs. 
My present purpose is to describe how the thickening 
process occurs in the older roots, for it is evident at a 
glance that the strong woody roots of a large tree have 
undergone many changes since they were the thin fili- 
form rootlets we met with in the young plant (see fig. 7). 
Not only have they increased in diameter, but they now 
consist almost entirely of wood, protected by a relatively 
thin, brown, corky covering, reminding one of certain 
kinds of bark. 
The first changes which take place when the young, 
thin roots begin to thicken are—first the piliferous layer 
dies away and the outer cells of the cortex turn brown; 
then a cylindrical layer of cork is developed in the peri- 
_cycle, and as this cork is impervious to water it cuts 
off the cortex from communication with the axis-cylinder, 
and consequently the cortex gradually shrivels up and 
is thrown off. 
Meanwhile active divisions have been going on in 
the cells immediately inside the phloém groups of the 
axis-cylinder (see fig. 5), and especially by means of 
