2 THE FACTORS [Part I 



many plants favours the exit of the water which has been absorbed, that 

 of some impedes it. 



Contrivances for expediting the exit of water are characteristic of 

 hygrophytes, or plants whose conditions of life exclude all danger of 

 desiccation, and in which a stagnation of the water, which brings nutritive 

 salts to the parts requiring them, may be feared. On the other hand, 

 difficulties in obtaining a supply of water lead to the formation of devices 

 for assisting absorption and limiting transpiration ; xcrophytes are provided 

 with contrivances of this kind. 



Wiesner l has pointed out another difference between hygrophytes 

 and xerophytes, but it does not appear to be fundamental, and in any 

 case requires further investigation. Xerophytes perish after two or three 

 days of continuous rain ; they are rain-avoiding, ombrophobons, whereas 

 hygrophytes are, as a rule, ombrophi/ous. This latter characteristic is 

 especially observable in the plants of very rainy climates ; for instance, 

 according to Wiesner, the vegetation of West Java (Buitenzorg) is ombro- 

 philous. On the other hand, there are in the moderately moist climate 

 of Central Europe ombrophobous hygrophytes, such as Impatiens Noli-me- 

 tangere. The feature to which Wiesner has drawn attention is externally 

 distinguishable thus : ombrophilons foliage is capable of being zvctied, om- 

 brophobous foliage is unzvcttablc. 



It is usual to designate the plants of moist localities as hygrophytes and 

 those of dry localities as xerophytes, but in this due attention is not paid 

 to the fact that the characteristics of organisms are physiological, those of 

 habitats are physical, and that there is no necessary connexion between 

 these two groups of characteristics. In reality, a very wet substratum 

 is quite dry to a plant if the latter cannot absorb water from it, whilst 

 a soil, that appears to us to be quite dry, may supply sufficient water to 

 many accommodating plants. A distinction sho?/ld therefore be made 

 betzveen physical and physiological dryness and between physical and physio- 

 logical moistness ; only the physiological characteristics need be considered 

 in plant-life and in geographical botany. A hygrophilous vegetation corre- 

 sponds to physiological moistness and a xerophilous vegetation to physiological 

 dryness. 



Xerophytes and hygrophytes are connected by transitional forms which 

 obscure the boundaries between them as two great oecological categories ; 

 it would therefore be useless to attempt to give the matter a statistical 

 basis. The unavoidable arbitrary convention adopted here does not, 

 however, actually involve so much confusion as might have been anticipated. 

 On the other hand, the constitution of a special category to include all 

 plants which are neither pronounced xerophytes nor pronounced hygro- 

 phytes would certainly add to the confusion. It appears, therefore, 



1 Wiesner, IV. 



