1 84 NATURAL SCIENCE. Sept.. 



in their own region, but also over the world as a whole. It is not 

 temperature which determines the presence or absence of Epiphytes. 

 They are absent from deserts which have a tropical temperature; 

 whereas shrubby representatives of this class are to be found on the 

 Himalaya, in regions where the average temperature is below that of 

 England. 



This demand for moisture is well illustrated by the distribution 

 of Epiphytes in England. Out in the open they are confined to a 

 few lichens, minute algae, few mosses, and a couple of sorts of liver- 

 worts. But in woods, where the air is damper, the number of these 

 epiphytic forms is much greater, and occasionally ferns are found 

 epiphytic. Where dampness of the air is combined with good 

 illumination, there is a still richer epiphytic flora; for instance, on 

 pollard-willows lining a river-bank there are many Epiphytes, some 

 of which attain the size of bushes (Rose, etc.). 



So, in the tropics, the moisture-laden jungles and primaeval forests 

 abound in Epiphytes ; on the other hand, the open savannahs have 

 but few. 



The second controlling factor is light. The demand for light 

 drives Epiphytes to the tops of the trees in a tropical forest. In 

 wandering through one of these forests it may seem as if there are 

 but few Epiphytes, till one espies their great aerial roots descending 

 to the soil and their fallen flowers and fruits dappling the ground. 



To a certain extent, light and moisture are wants which the 

 Epiphyte cannot obtain at the same time to the fullest extent. In 

 general, one may say that shade involves a moister atmosphere and 

 illumination a drier one; in addition, the stronger the light, the more 

 rapidly does a leaf transpire. Hence, in a tropical (American) forest, one 

 finds the Epiphytes ranged at different heights up the trees, and the 

 same forms constantly occur in the same positions. Low down in the 

 shade, on the trunks or thick lower branches, there are but few. 

 These are moisture-loving and shade-bearing species, especially 

 Hymenophyllaceae, Lycopodiaceas, delicate Pepcromias and orchids 

 (Zygopetalum), green bromeliads. On the tree-tops are forms which 

 can endure strong light, and are constructed so as to withstand the 

 danger arising from excessive transpiration (grey Tillandsias, thick- 

 leaved orchids without pseudo-bulbs, leathery polypodies). Between 

 these two extremes, on the branches below the tree-tops, the richest 

 and most varied epiphytic flora prevails. It is at this level that the 

 giants among Epiphytes reign, and countless orchids with pseudo- 

 bulbs flourish. 



Going out into the tropical American savannahs we find the Epi- 

 phytes far fewer. They are largely identical with those growing at 

 the extreme tops of the forest-trees. Only a few forms from the 

 middle stratum of Epiphytes occur, but none from the lowest stratum. 



Corresponding to the above-mentioned facts, we find that 

 densely-foliaged trees have few or no Epiphytes on them (Mango), 



