40 BIOLOGY 



it is practically certain that it is by chemotropism alone 

 that specific integrity is maintained among plants and 

 animals. By this we mean that fertilisation is made 

 possible by the similarity of the chemotropism within 

 the species, and that hybridisation is made difficult, if 

 not impossible, between dissimilar species, owing to the 

 difference in their chemotropism. 



The readiness with which lower organisms such as an 

 Amoeba or Spirillum respond to the stimulating effect 

 of food ; the fact that caterpillars hatched out on the 

 trunk of a tree creep up and reach the leaves upon 

 which they feed, show that chemotropism is important. 

 The reaction of plants and of animals to water, a 

 response which may be observed in nearly all forms of 

 life, is a variety of chemotropism. That it is important 

 can be easily shown and very neatly in the following 

 way : Remove the bottom from any small box and 

 replace it with fine meshed wire-netting, fill the box 

 with wet bog-moss, and plant some peas or beans in it. 

 Suspend the box, and in a few days it will be seen that 

 the roots have in response to gravity grown down 

 through the wire-netting. In a short time, however, the 

 roots will bend upwards and re- 

 enter the moss, as there is more- 

 moisture there than in the air, thus 

 showing that under certain cir- 

 Figure showing that Hy- cumstances the hydrotropic stimu- 

 drotropism is stronger lus is more powerful than the 



than Geotropism. geotropic stimulus. 



Another form of chemotropism, namely the response 

 to the effects of oxygen or oxytropism, is exhibited 

 by all living organisms. To high and low forms alike 



