THE ROOT 



67 



active power of discrimination, any substance that can pass 

 through the cell wall and its protoplasmic lining being taken 

 in, whether useful, unnecessary, or even harmful. These may, 

 however, be got rid of by excretion, as the superfluous water 

 taken in with dissolved minerals is exhaled from the leaves ; 

 or if incapable of passing out by osmosis, rendered harmless 

 and retained in the 

 form of the curious 

 "crystalloids" found 

 in various parts of 

 plants. But while 

 the kind of selection 

 exercised by vegeta- 

 ble cells implies no 

 power of choice, as a 

 matter of fact those 

 substances most 

 used by the plant in 

 carrying on its life 

 processes are ab- 

 sorbed in much 

 greater quantities 

 than others, being 

 transferred to parts 

 where growth or 

 other changes in the 

 plant tissues are go- 

 ing on, and there 

 used up in the work of nutrition, or excreted in part as waste 

 products. In either case their passage from cell to cell will 

 give rise to a continuous osmotic current in that direction, 

 and the absorption of new matter will go on in proportion to 

 the amounts used up. 



6i. Definition. — Tissue is a word used to denote any 

 animal or vegetable substance having a uniform structure 

 organized to perform a particular office or function. Thus, 



Fig. 73. — Roots of elm and sycamore contending for 

 isession of the soil on a rocky bluff on the Potomac. 



