Chap. IV] TROPICAL DISTRICTS CONSTANTLY MOIST 323 



which in this case are also supporting-roots, are constantly being formed by the 

 horizontally spreading crown, and thus its columned hall is built up. In most hemi- 

 epiphytes, if the absorbing-roots have no mechanical functions to fulfil they remain 

 soft and flexible. 



The collecting of humus that forms the characteristic of the nest-epiphytes 

 takes place in various ways. It is sometimes the roots that become 

 interwoven into a large sponge-like frame, which detains falling leaves and 

 the like, even after their decomposition, as in many orchids (Fig. 163) ; 

 sometimes the leaves of the epiphyte, arranged in a rosette, incline together 

 below to form a funnel, as in Asplenium Nidus (Fig. 139) — the plant 



Fig. 163. Grammatophyllum speciosum (Orchidaceae). A nest-epiphyte with negatively geotropic 

 roots, in the botanic garden at Buitenzorg. From a photograph by G. Karsten. 



figured had attained very large dimensions — and many other ferns, and 

 in the common Anthurium Hiigelii of the West Indies, an aroid greatly 

 resembling ferns in habit. Even in these cases, particularly in Anthurium 

 Hiigelii and in the orchids, there is usually a differentiation in the root- 

 system, as some of the roots which are not geotropic and are very strongly 

 constructed essentially contribute to form a framework, or act as anchoring- 

 roots, whilst numerous thin, lateral roots grow vertically upwards, so that 

 the lining of the root-nest appears to be bristling with countless needles 

 (Fig. 163). In contrast with nearly all other forms of roots, these thin roots 

 arc negatively geotropic, and this is oecologically connected with the fact that 

 nutritive material, especially water, comes from above and not from below. 



Y 2 



