i893- ON EPIPHYTES. 183 



utilising these, the plants in this group are, for the most part, merely 

 small herbs. 



In the next two groups the xerophytic structure is, on the whole, 

 less pronounced ; for these forms have advanced and have adopted 

 means to ensure a richer supply of food and water. In the second group 

 the roots have become differentiated into two sets — attaching 

 and nutritive. The latter descend to the ground. These forms 

 can be directly derived from the first group, and intermediate 

 types occur in which the nutritive roots reach the ground rather by 

 chance than in virtue of any strong geotropic properties {e.g., some 

 Cactaceae). 



The third group has for the most part roots differentiated into 

 attaching roots and absorptive roots which desert the substratum and 

 often are negatively geotropic. The roots or leaves, or both, make 

 provisions to collect humus. In the highest types not only are there 

 two sets of roots, but there is a differentiation in the leaves, some of 

 which are set apart to collect humus (species of Platyceriuvi, Poly- 

 podium, and Dischidia rafflesiana). This group, too, is derived directly 

 from forms like the first group. 



The second class — the Bromeliaceae — have been derived directly 

 from terrestrial bromeliads. They have become epiphytic in virtue 

 of the power which their leaves already possessed of absorbing nutri- 

 tive solutions. They have been formed utterly independently of, 

 and have pursued a course quite different to, that of the first class. 

 Yet it is striking to note how the effect of the environment has been 

 stamped on forms belonging to the two classes ; and to see how the 

 physiological anatomy of two forms may be identical and yet attained 

 in a totally different matter. For instance, comparing Tillandsia usneoides 

 with Mranthis, both are formed of pendent, green, assimilatory organs. 

 Yet the one is wholly made up a shoot-system, and the other, practically, 

 entirely a root-system. In both there is a general external absorptive 

 mechanism — numerous scales and velamen respectively. The nutri- 

 tive solution reaches the assimilatory tissue after first passing through 

 a la3er cuticularised except at certain points — the " exodermis," with 

 thin-walled passage cells, and the epidermis with the passage cells 

 under each scaly hair. Here and there free communication exists 

 between the atmosphere and the assimilatory tissue — by stomata in 

 one, and by air.-containing patches in the other. The assimilatory 

 tissue lies within the cuticularised layer. Rigidity is ensured in both 

 by sclerenchyma-fibres. In both there is a feebly-developed vascular 

 system, corresponding to the fact that each part of the whole surface 

 is capable of assimilating and of absorbing nutritive solutions, and 

 that hence conduction over long distances is not necessary. 



Distribution of Epiphytes within their own Region. 



Moisture is the first great factor which controls the distribution 

 of Epiphytes. This is true, not only of the distribution of Epiphytes 



