THE TEEE ITS BOOT-SYSTEM 91 



that the substances for making new roots and shoots, 

 new wood, and new leaves, &c., 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 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 phloem groups of the 

 axis-cylinder (see fig. 5), and especially by means of 



