340 VEGETABLE PHYSIOLOGY 



a limited period of the year. By preserving its leaves 

 green the plant can take advantage not 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, in such conditions as 

 sunlight or shade, drought or moisture, exposure to or 

 protection from cold winds, &c. 



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 

 feature about them is their aerial adventitious roots, which 

 are given off in some cases from every node of the stem, 

 so that each internode has its own supply. These are 

 often long cord-like structures, which are of some thickness, 

 often contain chloroplasts, and are either covered by a 

 special epidermal development, or give rise to dense masses 

 of root-hairs. In the first case, which is common among 

 epiphytic orchids, the epidermis is many cells thick, and 

 is known as the velamen. The cells are small trache'ids, 

 with curious reticulated or spiral thickenings, and are often 

 perforated. These peculiar trache'ids contain only air, and 

 the velamen has consequently a curious glistening greenish 

 appearance. The mass of trache'ids forms a kind oLspongy 

 cpjj3nng_Jto_th^^ 

 absorbing aqueous vapour from the moist atmosphere which 



