592 



ECOLOGY 



a 



medium facilitates absorption and consequently a dilute cell sap, 

 which in turn is thought to favor growth and luxuriance. Long 

 cultivation in a given medium results in " accommodation " to that 

 medium ; for example, the cell-sap concentration in a plant grown for 



a long time in a medium 

 of low concentration is 

 less than that in one simi- 

 larly grown in a medium of 

 high concentration. Fur- 

 thermore, a filament grown 

 for a long time in a dilute 

 solution reacts more quickly 

 to a concentrated solution 

 than one that has just been 

 produced from palmella 

 forms of long standing. 



As might be expected, sea 

 water induces the palmella 

 form of Stigeoclonium ; also 

 if a salt solution of low con- 

 centration is allowed to evap- 

 orate, the palmella form 

 gradually develops, owing to 

 the slow increase in concen- 

 tration. The palmella form 

 also may be induced by ex- 

 posure to transpiration (the 

 condition under which this 

 form probably develops on 

 trees), or to low tempera- 

 tures, even if the solution 

 is dilute. All changes in 

 form thus far noted may be 

 related to water, the pal- 

 mella form being produced in conditions where absorption is low (solu- 

 tions of high concentration or low temperatures) or evaporation high 

 (air cultures), or, in other words, where the cell sap becomes relatively 

 concentrated; the filamentous form, on the other hand, is produced 

 if absorption is high and transpiration lacking, that is, if the cell 



FIG. 86 1. Leaf variation as exhibited or- 

 dinarily by the mermaid weed (Proserpinaca 

 palustris) ; note the finely dissected water leaves 

 (w), the nearly entire air leaves (a), and the tran- 

 sitional leaves (/) just above the water surface. 





