14 Plant Life and Evolution 



with the different method of th^ir nutrition. With 

 few exceptions the cells of plants are surrounded by 

 a firm membrane which precludes motion except 

 where there are openings for the protrusion of the 

 protoplasm, a condition which sometimes occurs in 

 unicellular plants or those composed of a small num- 

 ber of cells. The tissues of the higher plants, made 

 up of these firm walled cells, are capable of only a 

 limited amount of motion. In animals the cell-wall 

 is much less evident in active tissues, and is often 

 not developed at all, thus allowing a much greater 

 degree of mobility in the cells than is the case in 

 plant tissues. 



Plants Immobile Organisms. The plant having 

 a constantly renewed food supply, CO 2 from the 

 atmosphere, and water and mineral compounds from 

 the earth, has no need to move from its position; 

 while the animal, obliged to move from place to 

 place in search of food, must be provided with 

 special organs of locomotion. If, however, an ani- 

 mal is so placed that it is provided with a constant 

 supply of food within its reach, it may often show a 

 plant-like immobility. This is seen in the case of 

 many parasites, and in such aquatic animals as 

 sponges, hydroids, and corals, which in their adult 

 condition sometimes curiously mimic vegetable 

 forms very closely, hence their old name of " zo- 

 ophytes." Many mollusks, like the oyster and mus- 

 sel, are also fixed in their adult condition. Such 

 stationary animals have developed ciliated organs 



