WASTES AND BY-PRODUCTS OF ORGANISMS 203 



the cells. Sand (silica) is thus found in the scouring rushes 

 and other plants ; and crystals of oxalate of lime are found in 

 hundreds of plants — for example, in the root of the jack-in- 

 the-pulpit and in other sharp-tasting plant parts (Fig. ^6). 



All these waste substances are useless to protoplasm, and 

 many of them are even injurious. But, separated as they are 

 from the living parts of the organism, they may nevertheless 

 be of some value to the plant as a 

 whole, or to the species, in some 

 special relation. Thus, the pigments 

 and odors of flowers may be of use in 

 relation to insect visits (see pp. 310- 

 314); or essential oils and tannins 

 may be of value in protecting plants 

 from animals and from bacteria or 

 fungi. 



235. Excretion in animals. The 

 one-celled animals excrete their wastes 

 just as they excrete carbon dioxid. In 

 the higher animals, those that have 

 blood and lymph, the wastes are dif- 

 fused into these conducting fluids, and 

 are then eliminated from the body 

 through special organs, for the most 

 part.^ 



236. The kidneys. In the human 

 body, which in this respect is typical 

 of the backboned animals, the kidneys are the special excretory 

 organs. Water and carbon dioxid are, as we have already 



Fig. 76. Crystals found in 

 plant cells 



/, in seed of castor-oil plant; 

 5, in bark of a tree ; j, in bulb 

 of squill. Crystals in plant cells 

 often represent an accumulation, 

 or a locking up, of superfluous 

 mineral matter 



^ To a comparatively slight extent the waste products of animals, like the 

 waste products of plants, are accumulated in some of the cells. Thus, many 

 of the pigments found in animals are no doubt to be considered as in the 

 nature of wastes deposited in the cells of the skin, or even in the interior of 

 the body. Much of the lime found in the skin of such animals as the starfish 

 or the sea lily, and the coral framework of the coral polyp, no doubt fall into the 

 same class. 



