EPIPHYTES 481 



materials is much impeded. The plants are generally of com- 

 paratively small size and bear thick, often rolled-up, leaves, 

 which are evergreen. The thick exterior and the general hard- 

 ness of the leaf are a response to and a defence against the cold ; 

 the rolled-up leaves bear on their protected faces abundant 

 stomata communicating with a relatively large spongy mesophyll, 

 so that transpiration, when possible at all, may be rapid. The 

 evergreen leaves also are an expression of the conflict against 

 the difficult}' of food absorption, which in such atmospheric con- 

 ditions is possible for only a limited period of the year. By pre- 

 serving its leaves green the plant can take advantage of every 

 opportunity afforded it. 



Some lowland plants show a similar response to their environ- 

 ment, 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. 



Epiphytic plants show some conspicuous modifications 

 of their structure in consequence of their peculiar habit of life. 

 They live usually 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 many 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 adaptation of form and structure. Perhaps the most 

 remarkable feature about them is their aerial adventitious roots 

 which are given off in some cases from everj' 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 with a special epidermal 

 development, or give rise to dense masses of root-hairs. In the 

 first case, which is common among epiphj'tic orchids, the epi- 

 dermis is many cells thick, and is known as the velainen. The 

 cells are small tracheids, with curious reticulated or spiral 

 thickenings and often perforated. The cells are empty, or con- 

 tain only air, and the velamen has consequently a curious glisten- 

 ing greenish appearance. The mass of tracheids forms a kind 

 of spongy covering to the root, and is capable of condensing and 

 absorbing aqueous vapour from the moist atmosphere which 

 usually surrounds it. The second case is illustrated by many 

 aroids, and the dense plexus of root-hairs borne upon the aerial 



VOL. II. I I 



