22 



INDIAN FOREST MEMOIRS. 



a scarcity of water owing to the slow movement of water by capillarity, and finally in a 

 swamp where there is a maximum of water but very little oxygen, and all this with very 

 slight changes in structure, the leaves of the swamp form still being clearly xerophilous. 



' If a deficiency of oxygen in the soil tends to depress root activity and thus diminish the 

 quantity of water which roots can obtain from the soil, we should expect a heavy non- 

 aerated soil like clay, or the water-logged soil of a swamp, to be 'physiologically dry, just as 

 a sandy soil, through the mere absence of water, may be physically dry, and consequently 

 we should expect to find the same species not infrequently on dry sandy soil and on heavy 

 clay, or in a swamp. This is the case and in India Bombay malabaricum, Odina Wodier, 

 Garuga pinnata and others are often found on physically dry, stony or sandy soils and 

 also in swampy localities, while Phyllanthus Emblica, JEgle Marmelos and others are often 

 found on dry, sandy, stony, soils, as well as on heavy clay. 



It must also be remembered that a clay soil may be physically dry, as well as physiolo- 

 gically dry, owing to the fact that the dense nature of the soil prevents the free movements 

 of water by capillarity and the soil may thus lose water by evaporation at the surface far 

 more rapidly than it can be replaced from the subsoil water below. 



It has also been pointed out above that xerophilous structures frequently serve equally 

 well as adaptations with reference to temperature conditions, as to moisture conditions,- 

 and it is therefore clear that we must be prepared to find one and the same species pro- 

 vided with a certain set of adaptations existing in a number of different habitats and 

 entering into the composition of a number of different communities. It may grow equally 

 well in different communities; it may be dominant in one and form a small proportion of 

 others; it may attain its highest development in one and show inferior dimensions in 

 another; it may reproduce itself readily in one and with difficulty in another, and so on. 



23. With particular reference to the vegeta- 

 tion of India it will probably be found convenient, at all events for the present, to confine 

 xerophj-tes, the word Hydrophyte to a true aquatic plant, as defined by Warming (paragraph 7 above) 

 Md°Hygro S - and then to subdivide terrestrial plants (including marsh plants) into Xerophytes, Meso- 

 phytes and Hygrophytes. 



It is necessary however to consider what is really the essential meaning of these terms. 

 OEcologists frequently overlook a fact of great importance, viz. that a xerophilous leaf- 

 structure, as a rule, merely indicates that transpiration is slow, that when assimilation is 

 proceeding the actual quantity of water vapour passed off from the leaf in a unit of time 

 is relatively small, but it by no means follows that the total quantity of water which a 

 plant with xerophilous leaves may be able to utilise during a period of say one complete 

 year is also insignificant. Owing to the fact that a plant with xerophilous leaves is 

 frequently able to remain vegetatively active during periods when another plant with 

 mesophilous leaves is obliged to cease activity, the actual quantity of water utilised in a 

 year, and the vigour of growth, as indicated by the dimensions attained, may be actually 

 greater in the case of the former than in the case of the latter. It is known that when the 



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Convenient 

 to classify 

 Terrestrial 



