8 INDIAN FOREST MEMOIRS. 



As the stomata are the openings through which gaseous exchange chiefly takes 

 place the protective coverings, e.g. of hairs, tend to occur chiefly on the- 

 under-surface, where the stomata in land plants mainly occur. 



(3) As cork forms an excellent protection against free gaseous exchange a thick 



corky bark is often characteristic of woody plants, the water current ascend- 

 ing in the wood thus being protected. 



(4) The loss of water by transpiration from a leaf depends greatly on the angle at 



which the leaf surface is exposed to the sun's rays, the loss, other things 

 equal, being greatest when the leaf surface is perpendicular to the direction 

 of the sun's rays and least when the surface is parallel to this direction, e.g. 

 when the leaf exposes its edges only to the direct rays of the sun. 



Thus motile leaves and leaflets are exceedingly common and characteristic 

 and in many species can be recognised at once by the presence of the so-called 

 pulvinus, pulvinule and glands. 



(5) The presence of ethereal oils and latex is believed to reduce transpiration, but 



in what way is not at present clearly understood. 



(6) Water-storing tissue is often characteristic. Plants obtain their water when 



they can get it, even when it is not needed for immediate requirements, and 

 then store it in these reservoirs where it is available for use when wanted. 

 The presence of such tissue also diminishes transpiration by checking gaseous 

 exchange and the conduction of heat. 



The presence of such tissue is often indicated by a succulent, or soft, and 

 somewhat spongy, texture and by the exudation of water when fresh tissues 

 are cut. 



The above will suffice to indicate a number of familiar plants to Indian Foresters 

 which, according to prevailing cecological ideas, must from their morphological and 

 anatomical characters be considered as fairly typical xerophytes, e.g. the desert Prosopis 

 spicigera with its extraordinarily deep-going root system ; the practically leafless species of 

 Tamarix, Casuarina, Euphorbia, Opuntia, Ephedra and Orthanthera; Calotropis with its 

 glaucous green leaves woolly beneath, Pinus with bluish, or dull green, foliage, the texture 

 of which is firm and hard and which is characterised by a well-developed cuticle and super- 

 ficial sclerenchyma : Moringa and Erythrina with thick corky bark ; A caeias with motile 

 leaflets; Euphorbia and Calotropis with milky juice, and many dry locality Rutacem with 

 ethereal oils, e.g. Feronia and JEgle. 



Mesophytes. 

 Warming notes that these plants " grow on soil which is of an intermediate character, 

 and is neither specially acid, nor saline, but is moderately moist, usually well ventilat- 

 ed, also rich in nutriment and in alkaline humus or in other organic constituents. Meso- 

 phytic communities occur in very diverse climates, near the Poles or on the equator, yet. 



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