g8 BOTANICAL GAZETTE [AUGUSi 



being the depth beneath the surface of the water; and all gases, 

 etc., and light which reach the cell from the exterior must pass 

 through this increased thickness of water. What different con- 

 ditions are here introduced, capable of modifying the life- 

 phenomena of the cell? Until we have a definite knowledge of 

 what these conditions are, i. ^., of the different physical con- 

 ditions involving the entrance of gases, salts, or light through 

 water measured by fractions of a millimeter or by centimeters, 

 we cannot hope to know much about the difference in the 

 environment of the cell in air and in water. 



Merely for the sake of convenience in discussion, we may 

 arrange what seem to be the possible factors acting upon aquatic 

 plants^under the following heads : nutrition, light, temperature, 

 gaseous content of water, substances in the water, and what 

 seems to have altogether escaped notice, the influence of the 

 water itself, either acting as a contact stimulus (stereotropism), 

 or by diluting the cell contents and protoplasm. Of course, such 

 a classification of factors is essentially superficial. It would be 

 quite erroneous, for example, to consider nutrition apart from 

 light which involves photosynthesis, or from temperature, or gas 

 exchange, or absorption of salts ; but it will enable us to discuss 

 the matter more systematically and with greater clearness. It 

 is to be understood that the experiments are given here only in 

 outline, 



I. Light. — A large number of experiments were conducted. 

 Plants were grown in the strongest sunlight available, and in 

 darkness, and in all stages between, the experiments always being 

 duplicated in water and in air. Plants were also grown in vari- 

 ous depths of water. The results were always uniform ; the 

 light apparently having no influence whatsoever on the character 

 of the developing shoot. When in the water it produced the 

 water form, and in the air the air form, regardless of the illumi- 

 nation. Plants with their tips the merest fraction of an inch 

 beneath the surface, in both strong and weak light, produced the 

 water form; and those with their tips just above the water pro- 

 duced the air form, no matter what the illumination {^fig* 7)- 



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