338 ZONES AND REGIONS [Pt. Ill, Sect. I 



These orchids, owing to the mode of spreading their organs, are principally- 

 adapted to light from above. The same holds good for many epiphytic ferns. 

 The widespread and very common Asplenium Nidus occurs under very various 

 degrees of illumination. Wiesner observed for it : L = 1/4 up to 1/38 (I maxi- 

 mum = 0-4-0-042). 



Epiphytes whose vegetative organs lie flat against the bark are adapted to light 

 from the front. Hence in the Buitenzorg garden, the otherwise common Taenio- 

 phyllum Zollingeri, Reichb. f., a small leafless orchid with assimilating roots pressed 

 close to the bark, does not grow in the orchid quarter, because the light from the front 

 is too weak. Wiesner has communicated the following, as a result of numerous ob- 

 servations made by himself, regarding the degree of light demanded by this plant : — 



LIGHT-REQUIREMENTS OF TAENIOPHYLLUM ZOLLINGERI, 

 REICHB. F. (after Wiesner). 



L. I maximum. I mean. 



Limits of development 1/3-1/32 0-533-0-050 0-166-0-015 



Most vigorous development 1/7-1/9 0-228-0-177 0-071-0-055 



Arrest resulting from insufficient intensity of light . 1/32 0-050 0-015 



Arrest resulting from too intense light . . . 1/2-1/3 0-811-0-533 0-251-0-166 



Flowers were observed with ..... 1/5-1/S 0-320-0-205 0-101-0-062 



Besides the epiphytes belonging to the Phanerogamae and the Pterido- 

 phyta which alone have been dealt with so far, the tropical virgin forest 

 also possesses others among Algae, Fungi, Lichenes, and Bryophyta, and 

 many of these plants, in particular some Hepaticae, also show a high degree 

 of adaptation to the substratum. Whilst the occurrence of such lower 

 cryptogams on the bark of trees is also exhibited in temperate forests, and 

 actually to a far greater extent than in tropical forests, their appearance as 

 epiphyllous forms, that is epiphytic on leaves (Fig. 170), is apparently 

 confined to the tropics. Epiphyllous forms are quite common features, 

 particularly on ageing leaves, in very humid rain-forests. 



The epiphytic plants on a tree in a virgin forest are not the same from 

 its base to its topmost branches, but exhibit a well-marked differentiation. 

 Low down on the trunk are many plants that are also terrestrial, such as 

 species of Hymenophyllaceae, Carludovica, climbing Araceae ; ascending 

 higher, these indifferent forms disappear, and the xerophilous character, 

 otherwise foreign to the rain-forest, increases with the increasing adaptation 

 to an epiphytic habit (Fig. 171), so that the epiphytic species met with, 

 which occasionally also occur on the ground, appear to be plants of very 

 dry stations, and, to some extent, of stations with strong illumination. The 

 epiphytes on the highest branches and consequently those that are most 

 insolated are identical with those that form the aerial flora of well-lighted 

 woodland and of the savannah of dry open districts. Hence after the partial 

 clearance of the forest, the epiphytes on the lower portions of the trees that 

 are left standing die, whilst those on the crowns gradually spread down- 



