226 PLANT PHYSIOLOGY 



is shown by the sporangiophores of some of the fungi. It 

 may well be that the fruiting organs of other plants depen- 

 dent upon dry air for the best dispersal of their seeds or 

 spores are also negatively hydrotropic. * 



Hydrotaxis is exhibited by many of the slime-moulds, for 

 example ^Ethalium {Fulligo). Stahlf shows that the dis- 

 tribution of these plants on and in the rotting wood, etc. 

 where they live, corresponds to the distribution of water 

 there; they are positively hydrotactic. 



The locomotion of such plants as are motile at any time 

 is dependent upon water, for this is the only medium 

 through which they can propel themselves. Many other 

 movements than those of locomotion are also dependent 

 upon water; the withdrawal of water, or variation in the 

 amount of water, may cause purely physical changes in cell- 

 walls or dead tissues which result in the exercise of me- 

 chanical force. These hygroscopic movements, such as the 

 opening of the peristome in mosses, the action of the elaters 

 of liverworts and Equisetum, the opening of fern-sporangia, 

 the curling of the awns of Geraniaceae, the splitting open 

 of pods, etc., etc., are free from the action of living proto- 

 plasm, and are mentioned in connection with irritable phe- 

 nomena only to make the difference clear. 



INFLUENCE OF OTHER SUBSTANCES THAN WATER 



In the whole range of chemical substances there are ex- 

 (tremely few which never in any way affect living protoplasm. 

 Many substances, however, affect protoplasm only mechani- 

 cally — as obstacles in the way of its growth or movement, 

 or, like the nitrogen of the air, which blows against and 

 subjects it to strain, etc. (For the effects of these see p. 

 189). Unless a substance is soluble in water it can af- 

 fect protoplasm in "no other than mechanical waj's. Of the 

 soluble substances some affect protoplasm favorably, others 

 unfavorably, others not at all. When an effect is produced, 



* Preliminary experiments on Fimhriaria California lead me to believe 

 that the sporophyte of this and of other liverworts (e. g. Anthoceros) 

 may be negatively hydrotropic. 



f Stahl, E. Zur Biologie der Myxomyeeten. Hot. Zeitung, 1884. 



