Chap. IV] TROPICAL DISTRICTS CONSTANTLY MOIST 319 



several meters in length, lie rootless and without any attachment, thrown 

 down, as it were, on to the ends of branches (Fig. 48) ; and Asplenium 

 Nidus frequently supports its funnels that are more than a meter in height, 

 in rows along thin liane-stems. 



According to their mode of life, epiphytes may be classified in four 

 groups : — 



1. PROTO-EriPHYTES. This is a very slightly homogeneous group, and 

 includes species that are compelled to acquire nourishment from the surface 

 of the supporting structure and from direct supplies from atmospheric 

 sources. 



2. Hemi-EPIPHYTES. These are epiphytes that germinate and pass 

 through their earliest development on trees, but subsequently become connected 

 with the ground by their roots, so that as regards their nutrition they 

 are subject to the same conditions as terrestrial plants, particularly root- 

 climbing lianes. 



3. Nest-epiphytes. This group is composed of species that by appro- 

 priate devices collect large quantities of humus and water. 



4. Tank-epiphytes. In these the root-system is developed only as an 

 auehoring-apparatus, or is entirely suppressed, so that the whole process of 

 nutrition is carried on by the activity of the leaves. The epiphytic Brome- 

 liaceae, at least in the tropics, are the sole representatives of this group ; 

 the New Zealand genus Astelia, consisting of lianes, appears to follow 

 them. 



Proto-epiphytes are frequently devoid of definite adaptive features. Thus, 

 for instance, small ferns that grow on moist fissured bark differ in no way 

 from those on the ground. In general, however, even the ferns of this group 

 are distinguished from the allied terrestrial plants of the evergreen rain- 

 forest by their decided xerophilous character, which the irregular and 

 scanty supply of water from their substratum sufficiently explains. The 

 epiphytes of the humid forest show protective devices against the loss of 

 water by transpiration, similar to those usual among plants that inhabit 

 physiologically dry stations. Such protection in this case is very rarely 

 afforded by hairs, much more frequently by a very thick cuticle and by 

 a sinking of the stomata into funnel-shaped depressions, most frequently, 

 however, by devices for the storage of the water, which at one time is 

 in excess on account of the rain, at another time is very scanty. Such 

 water-reservoirs may be developed in the form of a massive aqueous 

 tissue in the leaves, which then appear to be remarkably thick and juicy, as 

 in Peperomia, species of Aeschynanthus and other Gesneraceae (Fig. 16 a), 

 man}* Asclepiadaceae, or numerous water-tracheids are present, as in 

 the leaves of many orchids (Fig. 16), or special plant-members are converted 

 into water-reservoirs. Thus, the tuber-like structures possessed by so many 

 epiphytes — for instance numerous orchids, though not all of them — many 



