18 INDIAN FOREST MEMOIRS. 



The presence of ethereal oils also often appears to have a remarkable effect, at all 

 events in regard to low temperatures, and during the phenomenal winter of 1904-05, when 

 all indigenous woody plants suffered more or less severely in Dehra Dun, the plants which 

 showed the most conspicuous immunity were the exotic Cinnamomum Camphora and 

 Eucalyptus citriodora. 



The fact here emphasized that the so-called xerophilous adaptations may be of great 

 utility in enabling a plant to withstand extremes of temperature is obviously of practical 

 importance, inasmuch as xerophilous plants are in consequence the most suitable species 

 to introduce in localities not only where there is a scarcity of available water and a pro- 

 bability of damage by drought but also where there is a danger of injury by extreme heat 

 or cold. 



In this connection it must be remembered that the mere presence of water in the habi- 

 tat of itself directly tends to reduce the danger from extreme temperatures. The presence 

 of water vapour in the air tends to reduce both solar and terrestrial radiation and the 

 presence of water in the soil similarly tends to maintain an equable soil temperature. 



It is instructive to note that non-indigenous mesophilous and hygrophilous species of 

 bamboo which have been introduced and grown successfully in comparatively moist and 

 protected localities in Dehra Dun have been destroyed or greatly injured by frost in exposed 

 grasslands such as Ramgarh Tappar. 



Such a character as the possession of a thick corky bark, also, may often be of special 

 utility in the way of decreasing danger from fire damage. 



18. Even if such adaptations as the above can 

 be proved by experiment to be the ordinary response of the plant to the stimulus of a 

 variation in the water supply, this does not alter the fact tliat, in many cases at least, the 

 vital importance of such adaptations lies in this, viz. that they enable the plant which has 

 learnt to respond to this particular stimulus in this particular way to resist extreme 

 temperatures and thus to survive in the struggle for existence, while other plants which 

 responded in a different way, no less efficient as regards securing a sufficient water supply, 

 but failing to afford the necessary protection from temperature, perished. Seeing there- 

 fore that the main object of cecology is to explain " why each species has its own special 

 habit and habitat," it is obvious that we should avoid as far as possible laying undue stress 

 on a single factor, however important in itself, but which in a multitude of cases is probably 

 not the dominant factor responsible for the existence of forms of vegetation, as we now 

 find them in their natural habitats. 



importance 19. As indicating how an insistence on the 



primary importance of the water supply may quite possibly obscure the vital factor of 

 dominant importance, we may take the case of those evergreen trees which exist in regions 

 of moderate rainfall, such as Laurus nobilis, Ilex aqmfolium and Prunus Laurocerasus in 

 the Mediterranean region and species of Per sea, Laurus, Phoebe, Ilex and Myrica in the 

 Canary Isles, which are characterised by a more or less xerophilous type of foliage and 



[ 18 } 



