HOLE : SOME INDIAN GRASSES AND THEIR ECOLOGY. 17 



been the dominant factor responsible for the fact that that plant is only able to exist in 

 certain habitats and not in others, i.e. for its distribution as we now find it. 



Differences in the amount of available water for instance offer no obvious explanation of Necessity of 



.... avoiding 



such remarkable facts as that, in the deciduous Indian forests, on the same soil and in the supra-maxi- 

 same climate, some species like Schleichera trijuga, Bassia latifolia, and others, come into turo may 

 full vegetative activity in the hottest dryest season of the year, while the majority, like Teak, Me'fofa" 8 

 wait until the near approach of the monsoon insures an easy and plentiful water supply, periodicity. 

 If the water supply is here the factor of dominant importance what advantage can have 

 been gained by the plants which first showed a tendency to become active in the hot 

 season, which can have accounted for the evolution and survival of such species ? No such 

 advantage can I think be shown from the point of view of the water supply, but there is a 

 real advantage from the point of view of temperature. Transpiration undoubtedly reduces 

 the temperature and it is quite possible that some species avoid the risk of a rise of tempera- 

 ture above the supra-maximal by transpiring actively during the most dangerous season of 

 the year, although other species are able to exist with different periods of activity, owing 

 to their being less sensitive to injury from high temperatures, to their possessing more 

 adequate protective arrangements during the resting season, and so on. 1 



A most remarkable fact which possibly indicates the importance of active transpira- 

 tion during the hottest season is that in the swamps of the Dehra Dun valley? at an 

 elevation of less than 2,000' above Mean Sea Level, the following temperate species occur, 

 which are usually found in the Himalayas at an elevation of 4,000' to 8,000', and this may 

 be due to the fact that vigorous transpiration is possible in the hot season owing to the 

 abundance of water available : — 



Hedera Helix which often ascends to 9,500' in the Himalayas and Quercus incaria. 



17. Again if we consider the danger from xeropiuious 



Characterise 



tare condi- 

 tions. 



infra-minimal temperatures, no Forester who has had experience of the forests of Central tics, general, 

 and Northern India would be inclined to doubt the immense importance of frost as a factor regarded as 

 influencing the distribution of trees and it is a remarkable fact that almost all the adapta- to tempera! 

 tions noted in paragraph 7 above as tending to reduce transpiration and to be characteristic 

 of xerophytes also tend to reduce liability to injury by frost and low temperatures. The 

 reduction of the radiating leaf surface and the movements of leaves and leaflets reduce 

 radiation, while protective coverings of sclerenchymatous tissue, cuticle, cork, wax and 

 hairs all protect the living tissues from contact with chilled external air. 



It should be noted also that two of our most typical Indian xerophytes are also two of 

 our most frost-hardy species, viz. Acacia Catechu and Zizyphus Jujuba. Just as these 

 adaptations moreover may be of service in affording protection from extreme cold, to 

 scarcely less an extent may they be of service in affording protection from extreme heat. 



1 Plant-competition is also a factor which may possibly be responsible for the evolution of the remarkable periodicity noted above. 

 Species capable of growth at seasons which are unfavourable for tbe majority of plants may, on this account, be able to survive and hold 

 their own in the struggle for existence with more vigorous plants, the period of vegetative activity of which, however, is either shorter 

 or occurs altogether at a different season. 



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