230 PLANT PHYSIOLOGY 



seasonally, are far more susceptible to changes in the den- 

 sity of the water. It has been suggested that the recourse 

 of fresh water algae to their various modes of reproduction 

 (see pp. 268-9) may be a reaction to osmotic differences 

 in the water in which they live,* but this is by no means 

 assured. 



Certain substances, nutritious and other, stimulate the 

 growth and other activities of plants as of animals. Small 

 amounts of poisonous matters, such as ether, t metallic 

 salts, cocaine, and morphine]: act as stimulants. The prin- 

 ciple governing the action of these substances is the same 

 as that controlling the use of tea, coffee, and tobacco by 

 human beings, for all of these drugs are dangerously de- 

 pressing poisons when taken in concentrated form. Alcohol 

 may, in both animals and plants, serve as a source of 

 energy by oxidation and thereby stimulate the organism. 



One word must be said about the startling claim that 

 definite chemical stimuli may replace the male elements in 

 sexual reproduction. Loeb reports that he has reared sea- 

 urchins through the earlier larval stages from unfertilized 

 eggs by subjecting the fggs to suitable chemical stimuli. 

 From this he argues that the main function of the sperm 

 is to supply the eggs with certain compounds indispensable 

 to further development, and that when, no matter by what 



* Livingston, B. E. Nature of the stimulus which causes change of 

 form in polymorphic algae, Bot. Gazette, vol. 30, 1900. Further notes 

 on the physiology of polymorphism in green algae. Bot. Gazette, vol. 32, 

 1901. 



t Townsend, C. O. The correlation of growth under the influence of 

 injuries. Annals of Bot., XI.. 1897. 



t Richards, H. M. Die Beeinflussung des Wachsthums einiger Pilze durch 

 chemische Reize. Jahrb. f. wiss. Bot., Bd. 30, 1897. See also Raciborski, 

 M. Uber den Einfluss ausserer Bedingungen auf die Wachsthumsweise des 

 Basidiobolus ranarum. Flora, Bd. 83, 1896. 



Atwater. W. O. Metabolism of matter and energy in the human body. 

 Bull. U. S. Dept. Agriculture, Washington, 1901. 



Loeb, J. On the nature of the process of fertilization and the artificial 

 production of normal larvae (plutei) from the unfertilized eggs of the 

 sea-urchin. On the artificial production of normal larvae from the un- 

 fertilized eggs of the sea-urchin (Arbacia) , Amer. Journ. Physiol., vol. III., 

 1899-1900. 



