

148 



BOTANICAL GAZETTE 



[FEBRUARY 



cited as an especially good example; its leaves are exposed alter- 

 nately to fresh and salt water, but its roots, being covered by mud, 

 are exposed to comparatively little change in salinity. The theory 

 of adaptation might lead us to expect that the leaves of such plants 

 would be much more tolerant of fresh water than the roots. This 

 expectation is most strikingly confirmed by experiments, which 

 show that the root cells of these plants are killed by fresh water in a 

 few minutes, while the leaf cells can stand exposure to fresh water 



for several hours. 



the argument must 



make experiments with specimens of eel grass taken from 



remote 



opportunity 



mouths of streams, where no 

 water occurs. In these plants 



find the same differences between root and leaf with resp 

 ir ability to withstand fresh water that we find in plants gr 

 the mouths of streams. 

 We must suDDose, therefore, that characters which seem 



in this case nresent from 



must be ascribed to entirely diff 

 so true of many cases which at 



Doubtless 



instances of adaptation. 



7 



5 is much significance in the fact that leaf cells may 

 much longer exposure to fresh water than the roo 



same 



[it. One might be inclined to explain this by differ- 

 1 wall rather than by differences in the protoplasm, 

 the cell wall in the root is usually more permeable 



the 



that 



the case here, however, for when leaf cells and root cells are placed 

 side by side in hypertonic sea water, they are plasmolyzed with 

 equal rapidity, and when replaced in ordinary sea water they recover 

 at the same rate; this shows that their permeability to water and 

 to the salts in the sea water is about the same in both cases. 



Another consideration shows that the difference in the behavior 

 of the cells cannot be due to differences in their permeability to 

 water. This is the fact that death is not primarily due to absorp- 



7 Experiments with other species growing at the mouths of brooks showed that 

 individuals which have had no opportunity for adaptation to fresh water show a 

 great tolerance of it. 







