346 VEGETABLE PHYSIOLOGY 



cold the closing of these almost hidden stomata guards the 

 plant from the evaporation, which, if unchecked, would 

 lead to a loss of heat that might be fatal to it. The meta- 

 bolism being reduced by the low temperature, the contents 

 of the air reservoirs suffice for such interchanges of gases as 

 are imperative, and for the coincident exhalation of watery 

 vapour by the protoplasts, but as these contents are very 

 slowly renewed the total evaporation is but slight. When, 

 on the other hand, for a part of the year the temperature 

 is high, the spacious reservoirs provide for a very rapid 

 transpiration as soon as the stomata are open, a very 

 large spongy mesophyll abutting on them (fig. 144). The 

 evergreen leaves also are an expression of the struggle 

 against the difficulty of the absorption of food materials, 

 which in such atmospheric conditions is possible for only 

 a limited period of the year. By preserving its leaves green 

 the plant can take advantage riot only of the light of summer, 

 but also of those bright sunny days which occur occasionally 

 during the cold season, and thus improve every opportunity 

 afforded it. 



Some lowland plants show a similar response to their 

 environment, the form and structure of different individuals 

 of the same species varying to a certain extent, according 

 to their advantages or the reverse, under such conditions 

 as sunlight or shade, drought or moisture, exposure to or 

 protection from cold winds, &c. 



Epiphytic plants show some conspicuous modifications 

 of their structure in consequence of their peculiar habit of 

 life. They usually live upon the surfaces of trees, to which 

 they cling by various means, but from which they derive 

 no nourishment except such as is afforded by accumulations 

 of debris, &c., upon the trunks. They are not parasitic, 

 but merely live upon the tree as other plants grow upon 

 rocks or cliffs. Mosses and Liverworts are very largely 

 epiphytic, as are certain species of Phanerogams ; the 

 latter are very specialised forms, and show most adapta- 

 tion of form and structure. Perhaps the most remarkable 



