190 



PLANT PHYSIOLOGY 



plied to them, whether this be upward or downward. This 

 phenomenon is called rheotaxis, * but it is not certain that 

 it is not a response to obscure chemical stimuli rather 

 than to a current of water merely. Rheotaxis and rheo- 

 tropism are therefore probably distinct phenomena. The 

 significance of rheotropism is not understood. 



FKJ. 13. 



Figure 13. PosteMa Palmspformis. Sea-palms at Point Lobos, near 

 Monterey, California. Height about 2 feet. Photograph by Dr. W. A. 

 Shaw. 



Waves and tides have not been studied experimentally in 

 their mechanical relation to plants. It may be inferred, 

 perhaps, that they produce movements which stimu- 

 late the plants exposed. Certainly plants which are to 

 withstand the pounding of the waves must grow propor- 

 tionally resistant. Is this simply a case of the survival 

 of the accidentally toughest and fittest, or do tide and surf 

 plants irritably react to the rude stimuli to which they are 



Stahl, E. Zur Biologic der Myxomyceten. Bot. Zeitung, 1884. 



