48 BIOLOGY: GENERAL AND MEDICAL 



carry the pollen from the anthers to the stigma, or by 

 insects and birds that carry the pollen grains from flower 

 to flower, chemotropism is shown by the inability of the 

 pollen grains of heterologous species and the ability of 

 those of homologous species to grow into the stigma and 

 descend to the ovules below. 



The general explanation of the ease with which specific 

 integrity is maintained, and hybridization made difficult 

 in both plants and animals, probably rests upon condi- 

 tions of positive and negative chemotropism existing 

 between the male and female sexual elements of similar 

 and dissimilar species. 



Among animals whose ova and spermatozoa are liber- 

 ated freely in the water in which they live, the union of 

 the cells must be determined by chemotropic influences. 

 Among higher animals in which the spermatic fluid is 

 emitted into the sexual organs of the female, it must 

 be chemotropic influences that govern the movements 

 of the spermatozoa during their progress toward the 

 ovum, and finally determine their entrance into it in- 

 stead of into other cells with which they may come into 

 contact during the interval. 



Chemotropism is specialized in the higher animals as 

 taste and smell. 



A. Sitotropism or Reactions toward the Stimulating 

 Influences of Food. 



This can be differentiated with difficulty from simpler 

 forms of chemotropism. The wear and tear of the cyto- 

 plasm of the cells of living organisms brought about 

 through their activities, makes it imperative that means 

 be provided for reintegration of the impoverished tissues. 

 A food supply, therefore, becomes imperative. 



The simple character and almost universal distribu- 

 tion of the foods of plants make it unnecessary for them 

 to manifest sitotropic activities. The more complex 

 food requirements of animal organisms determine that 

 the majority pursue or catch their food. 



