168 COLONIAL REPORTS MISCELLANEOUS. 



In nature, usually a combination of all these factors or the 

 greater number of them has to be faced by xerophilous plants 

 in the localities they inhabit, and their adaptations to these 

 conditions have brought about the following adaptations of 

 structure. 



Reduced transpiration is brought about by a decrease in the 

 (ranspiring surface of the leaves for the same mass of plant 

 substance by the formation of a thick cuticle on the exposed 

 surfaces of the leaves, by the reduction in size of the intercellular 

 air spaces of the leaf, by a sinking of the stornata, and by the 

 presence of felted hairs containing much air. 



In addition to this, Xerophytes are often characterised by the 

 presence of water-storing cells (double epidermis, aqueous tissue, 

 mucilage cells, &c.), an increase in the vessels and scleren- 

 chyrnatous tissues, lengthening of the palisade cells of the leaf, 

 a copious root system, underground stems, the power to orient 

 their leaves so as to avoid insolation at the hottest and brightest 

 time of the day and, in the case of those possessing pinnate 

 leaves, the power to open or close them according to the degree 

 of illumination ; the production of thorns from reduced leaves 

 and shoots, by being ombrophobous or rain-avoiding, and by the 

 formation of thicker and more fleshy or proportionately more 

 leathery leaves, or by the reduction of the latter to rudimentary 

 organs, in which case the axes become rich in chlorophyll. 



These modifications are chiefly confined to the leaves, which 

 are the organs most concerned with the act of transpiration; 

 modifications of the axes (stems), however, are not rare. Those 

 of plants belonging to the Cactus family become short and thick 

 and even spheroidal, and are filled with a mucilaginous sap. 

 Similarly the stems of some Euphorbias become succulent, and 

 even those of gigantic trees like the tropical African Baobab 

 tree (Adansonia digitate^ acquire a monstrous, gouty shape, 

 and the wood, becoming 1 soft, juicy, and spongy, forms a vast 

 water-reservoir. 



Water-cells have a similar function; they rapidly fill up when-, 

 ever the water supply is temporarily increased (during rainy 

 weather) and yield their contents to the assimilating cells as 

 the supply of moisture becomes reduced. 



As regards ombrophoby, Xerophytes perish if they are exposed 

 for a few days to continuous rain, and their leaves cannot be 

 wetted except after prolonged exposure to water. 



The most important natural habitats where Xerophytes thrive 

 are : 



(1.) Districts with a dry substratum, dry air, occasional or 

 persistent heat, and intense illumination. 



(2.) The surfaces of the bark of trees (with reference to the 

 growth on it of epiphytes), rocks, &c.. where, owing 

 to a want of sufficient depth of the substratum, any 

 moisture deposited on them is rapidly evaporated. 



(3.) Sandy, gravelly, and other similar soils which, owing 

 to their permeability, are rapidly dried up. 



