IRRITABILITY 231 



means, these compounds are supplied, development will go 

 on., This may be true, but that it is true is far from proved. 

 There are well-known cases of parthenogenesis among both 

 animals and plants. From these one might infer either that 

 no eggs require fertilization in order to develop, or that 

 these eggs are chemically stimulated to develop by the sub- 

 stances in air or water. Both of these inferences would be 

 unjustified. Sea-urchin eggs may not develop partheno- 

 genetically when let alone, though they may do so when 

 chemically stimulated. But it is a far cry from Loeb's 

 brilliant experiments to the generalization that fertilization 

 is principally chemical stimulation. In many cases, the 

 structure of the egg must be completed by the material of 

 the sperm before normal development can begin. This, in a 

 sense, is chemical stimulation, but it is also a morphological 

 process. Fertilization, in most cases at least, consists in the 

 completion of the structure of the egg as weU as in the 

 stimulation of its living protoplasm to grow, etc. 



The direction of growth, as well as the rate and kind, is 

 influenced by other substances than water. This response of 

 the living plant to external stimuli is called chemotropism. 

 It is well known that roots grow toward, around, and into 

 especially fertile clods of earth, leaving the less nutritious. 

 The distribution of oxygen, carbon-dioxide, and possibly of 

 other gases in the soil (e. g. illuminating gas leaking from 

 underground pipes) affects the growth of the roots. * They 

 will not grow far down into water ; they will turn up when- 

 ever their supply of air faUs below the optimum. They will 

 grow toward the sources of many gases, but so soon as 

 they reach a point where the amount of gas becomes at all 

 considerable, they will bend and grow away. 



The chemotropic sensitiveness of hyphaef and of pollen- 

 tubes J is marked and important. Miyoshi finds that the 



* Molisch. H. Fber die Ablenkung der Wurzeln von ihrer norraalen 

 Wachsthumsrichtung durch Case (Aerotropismus) . Sitzb. d. Akad. d. Wiss.. 

 Wien. Bd. 91. I.. 1884. 



4 Miyoshi. M. Uber Chemotropismus der Pilze. Bot. Zeitung 1894. 



i Miyoshi, M. Uber Reizbewegungen der Pollenschlanche. Flora. Bd. 

 78. 1894. Lidforss, B. fl)er den Chemotropismus der Pollenschlauche. 

 Ber. d. Deutsch. Bot. Ges. Bd. 17. 1899. 



