486 The Physiology of Plants BOOK in 



Molisch showed that roots curve towards free oxygen but 

 away from carbon dioxide, while pollen-tubes tend to avoid 

 the former. In 1894 Miyoshi published some extended 

 researches upon the same question, giving many instances 

 of sensitiveness of various fungi to different stimulating 

 substances. He showed that the penetration of a parasite 

 into its host is mainly due to chemotactic stimulation. 

 He also gave a more complete account of the behaviour 

 of the pollen tube in its progress from the stigma to the 

 ovule, and showed that cane sugar secreted by the micropyle 

 plays an important part in the guiding process. Correns 

 showed, in 1896, that both chloroform and ammonia 

 cause curvature of tendrils. In 1900 Stahl published further 

 researches on the Myxomycetes, in which he showed that 

 variations in the concentration of the solutions of the 

 various stimulants are appreciated and responded to. 



OTHER KINDS OF SENSITIVENESS 



During the last twenty years of the century many other 

 forms of irritability were discovered. The power of appre- 

 ciating differences of temperature, associated so long ago 

 with the opening and closing of flowers was found to be 

 capable of causing curvatures. In Wiesner's work on helio- 

 tropism he showed that the infra-red rays act in the same 

 way as those of the visible spectrum . This form of irritability 

 was named thermotropism by Van' Tieghem. Wortmann 

 found in 1883 that seedlings of cress and of maize, and the 

 sporangiophores of certain Phycomycetes curve towards a 

 hot plate which is giving off heat-rays only ; in 1884 Barthe- 

 lemy saw that roots of Hyacinths act in the same way, 

 while the leaves of the plant curve away from a heat 

 source. In 1890 Vochting showed the occurrence of positive 

 thermotropism in the peduncle of Anemone. 



Curvature of roots under the influence of moisture, which 



