1 6 ZOOLOGY. 



locomotion and other forms of mechanical work, or light, or 

 electrical energy may be secured by the oxidation. So in 

 protoplasm, various types of energy may result from the tear- 

 ing down of the complex substances. Among these are animal 

 heat, motion, nervous energy and electrical energy. 



26. Secretion and Excretion. As a result of the constructive and 

 destructive work already mentioned as characteristic of protoplasm cer- 

 tain substances, not themselves protoplasm, may be produced. If these 

 products are of further use in the animal economy, they are usually 

 described as secretions; if they represent the final reduction in the process 

 of tearing down, they are called excretions. Such materials may be de- 

 posited either within the protoplasm or at its surface. In the latter case 

 it may be deposited in a uniform sheet and produce a protective mem- 

 brane (cell wall). The presence of such a covering to the protoplasm 

 very materially modifies all the elementary activities which have been 

 described. 



27. Demonstrations. The teacher should make microscopic demon- 

 strations of secretions and excretions : as starch grains formed in the 

 leaves of plants ; fat in adipose tissue ; cell-walls in plants ; crystals in plant 

 cells (see Botanies) ; intercellular substance in cartilage or bone. , 



28. Supplementary Topics for Library Work. Find and examine 

 some of the classic definitions of life. Examine more completely the 

 theories of protoplasmic architecture. In what ways would the presence 

 of the cell-wall bring about modifications of the protoplasmic activities? 

 Give an account of experiments showing the effect of some of the more 

 important stimuli on protoplasm (as light, heat, electricity). What of the 

 external conditions are so important as to merit the term "primary con- 

 ditions of life " ? Why may protoplasm be described as chemically un- 

 stable? Compare oxidation in the protoplasm with oxidation in ordinary 

 combustion. 



29. Summary. i. Scientists are not agreed whether life 

 is merely the action of the ordinary chemical and physical 

 forces in connection with a peculiar substance, or represents 

 these, guided by a type of energy of a higher order. 



2. Protoplasm, a chemical compound of exceeding complex- 

 ity and instability, is the " physical basis of life." Differences 

 in various living things are probably due to differences in the 

 chemical and physical structure of the protoplasm of which 

 they are composed. 



3. Owing to the unstable character of the protoplasm it is 

 readily acted upon and changed by external forces; and the 



