No. 510] PROBLEMS IN PLANT ECOLOGY 



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and seemingly saner ecological classification of plant 

 structures. 



The work of Witte seems to show that plant forms may 

 be either obligate or facultative, that is that they may be 

 inherent or may be subject to environmental control. In 

 a xerophytic flora, such as that studied by Witte in 

 Sweden, there are many plants of dwarf habit. Experi- 

 ment shows that these dwarfs behave very differently in 

 mesophytic conditions. Some develop into tall and 

 luxuriant plants, while others remain as truly dwarf as 

 in xerophytic soil. The dwarfness of the former is facul- 

 tative, of the latter obligate. The former is clearly a 

 response or reactive xerophyte, owing its form to its sur- 

 roundings; the latter appears to be an inherent or con- 

 genital xerophyte, the cause of whose development is as 

 yet unknown. It is probable that further experiment 

 along these lines will show that many other plant forms 

 apparently identical are most unlike in nature and origin. 

 Time will permit but one further illustration. In meso- 

 phytic forests one sometimes finds plants that appear to 

 be xerophytes. A notable example of this is seen in 

 many species of Begonia and Peperomia that grow in the 

 depths of the rain forest. Both in external aspect and 

 in all the details of leaf structure such plants appear 

 xerophytic; it seems almost certain that these plants are 

 congenital xerophytes, species whose xerophytism is 

 stamped upon them to such an extent that age-long ex- 

 posure to the moisture of the rain forest has not de- 

 stroyed it. Whether congenital forms in the final anal- 

 ysis differ in any radical degree from reactive forms can 

 not now be told ; it may be that all plants are or have been 

 plastic, but it is clear that the recognition of the double 

 possibility in the interpretation of plant structures is at 

 present advantageous. 



A minor but necessary matter is that of terminology. 

 One of the unfortunate features about language is that 

 expressions lag behind ideas. Long ago we knew that 

 the heart is not the seat of the emotions, but our lan-ruauv 



