rials are regular by-products of metabolism which we may consider as 

 "wastes". And they are stowed in plant cells, rather than stored, instead of 

 being pushed out of the system, or excreted, much as useless rubbish is 

 stowed away in the cellars and attics of many homes. 



Excess of mineral matter absorbed from the soil is separated out of living 

 cells and precipitated as insoluble compounds. Thus crystals of oxalate of lime 

 are found in hundreds of species — for example, the horse-radish, the root of 

 jack-in-the-pulpit, and other sharp-tasting parts (see illustration, p. 215). 



We usually classify the most common organic wastes in relation to their 

 possible uses to us, as below: 



Human Uses of Organic Plant Wastes 



Pigments. Direct enjoyment of color in flowers, fruits, leaves, wood, etc. Extraction of 

 dyes for use on fabrics. 



Essential oils. Direct enjoyment in fruits and flowers; spices. Extraction for perfumes, 

 seasoning foods, candy, etc. 



Gums and resins. Adhesives, waterproofing, protection of materials against insects and 

 fungi, sealing joints. 



Tannins. Chiefly for tanning leathers; drugs. 



Alkaloids. Poisonous generally; used as drugs — morphin, quinin, atropin, cocain, caffein, 

 digitalin, etc. 



Although these waste substances are useless to protoplasm, they may be 

 of some value to the plant as a whole, or to the species, in some special rela- 

 tion. Thus pigments and odors of flowers may be of use in relation to insect 

 visits, or essential oils and tannins may be of value in protecting plants from 

 animals and from bacteria or fungi. 



Excretion in Animals To a comparatively slight extent waste prod- 

 ucts of animals are accumulated in some of the cells, like the waste products 

 of plants. Thus some of the pigments found in animals are no doubt to be 

 considered as 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 and the sea lily and the coral framework of the coral polyp fall into 

 the same class. Small quantities of lead are found in the skeletal tissues. 



One-celled animals excrete their wastes just as they excrete carbon di- 

 oxide, by diffusion. In animals that have blood and lymph, wastes diffuse 

 into these conducting fluids and for the most part are then eliminated from 

 the body through special organs. 



How Are Wastes Removed from Animal Bodies? 



The Lungs and the Skin Water and carbon dioxide are excreted from 

 the lungs, as well as small quantities of urea and possibly other organic sub- 

 stances (see page 187). A certain amount of waste gets into the intestine 



216 



