DISTINCTIONS BETWEEN ANIMALS AND PLANTS 169 



with all spores or other reproductive cells, and then to pre- 

 vent the entrance of any more germs. An experiment in this 

 line can be performed by heating an extract in a flask, closed 

 by a plug of cotton, until all germs have been killed. Such a 

 solution then will not even produce the bacterial growths that 

 cause decomposition, for the cotton plug prevents the entrance 

 of any dust. It is now established that all organisms at present 

 on the earth are generated only by their like; that life only 

 comes from life, and protoplasm from preexisting protoplasm. 



202. The distinctions between animals and plants. Plants 

 in general are distinguished from animals by two important 

 peculiarities. 



First. The presence of chlorophyll, or equivalent pigments, 

 enables the plant to manufacture its own food by photosynthesis 

 in the interior of its own cells. Animals require foods already 

 manufactured by other animals or plants, and this food is ab- 

 sorbed from without the cell. 



Second. Plants, when growing, are generally stationary, with 

 a firm, widely expanded, rigid structure, while animals are more 

 rounded, compact, and yielding. These differences are deter- 

 mined by the fact that the protoplasts of plants are inclosed 

 in cellulose compartments. The cell wall gives to plant struc- 

 ture a degree of stiffness which greatly limits or almost pre- 

 vents movement, but the individual protoplasts of plants have 

 all the characteristics of life in common with animal cells, 

 sensation, movement, and the powers of growth, repair, and 

 multiplication. 



203. Some organisms of doubtful position. Several groups 

 of lowly organisms have characters which are in part plant-like 

 and in part animal-like. We shall consider briefly only one of 

 these groups, the flagellates. 1 



1 Another large group of doubtful position is the slime molds, or 

 Myxomycetes, more frequently included among plants than among ani- 

 mals, but too special for this account. See Macbride, The North American 

 Slime Moulds, 1899. 



