XVII DEFINITIONS 179 



diffusion from the general surface of the organism into the 

 surrounding medium. 



This character also is of some general inportance. The 

 large majority of animals possess a special organ of excretion, 

 plants have nothing of the kind. 



Another difference has to do with the general form of the 

 organism. Paramoecium has a certain definite and constant 

 shape, and when once formed produces no new parts. 

 Vaucheria and Mucor are constantly forming new branches, 

 so that their shape is always changing and their growth can 

 never be said to be complete. 



Finally, we have what is perhaps the most obvious and 

 striking distinction of all. Paramcecium possesses in a con- 

 spicuous degree the power of automatic movement ; in both 

 Mucor and Vaucheria the organism, as a whole, exhibits no 

 automatism but only the slow movements of growth. The 

 spores and sperms of Vaucheria are, however, actively 

 motile. 



Thus, taking Paramoecium as a type of animals, and 

 Mucor and Vaucheria as types of plants, we may frame the 

 following definitions : — 



Animals are organisms of fixed and definite form, in which 

 the cell-body is not covered with a cellulose wall. They 

 ingest solid proteinaceous food, their nutritive processes 

 result in oxidation, they have a definite organ of excretion, 

 and are capable of automatic movement. 



Plants are organisms of constantly varying form in which 

 the cell-body is surrounded by a cellulose wall ; they cannot 

 ingest solid food, but are nourished by a watery solution of 

 nutrient materials. If chlorophyll is present the carbon 

 dioxide of the air serves as a source of carbon, nitrogen is 

 obtained from simple salts, and the nutritive processes 



N 2 



