THE ROOT 57 
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 
“erystalloids” 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- Fic. 73. — Roots of elm and sycamore contending for 
ing on, an d th. ere possession of the soil on a rocky bluff on the Potomac. 
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. 
61. 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, 
