THE TREE ITS ROOT-SYSTEM. 91 



that the substances for making new roots and shoots, 

 new wood, and new leaves, etc., are constructed. It is 

 in 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 ob- 

 tained from the air in the chlorophyll corpuscles, and 

 consequently a larger yield of plant-forming materials 

 is possible to meet the demands of the ever-growing 

 organs. 



My present purpose is to describe how the thicken- 

 ing 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 

 filiform 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 phloem groups of the 



