i8 9 3- ON EPIPHYTES. 



*77 



the wet season comes on, the intercellular system being lined with 

 mucilage, absorbs water which gradually replaces the air. The 

 swollen petiole is a water-reservoir calculated to meet the demands 

 of the lamina. This is proved by the following experiment : If a 

 whole leaf be isolated and left to dry for fifteen days it remains 

 unchanged in appearance; but if the lamina alone be treated thus, 

 it fades and shrinks in a few days. 



Among the Orchidaceae, the arrangements for the storage of 

 water are varied and characteristic. Some possess leaves with well 

 developed broad tracheides, the function of which is to store water. 

 In others, the water is retained in pseudo-bulbs. Experiments 

 similar to those on Philodendron prove that the latter are water- 

 reservoirs. For this reason, orchids with pseudo-bulbs usually have 

 thin leaves ; whereas, orchids with fleshy leaves are devoid of pseudo- 

 bulbs. Only a few orchids have thin leaves and no pseudo-bulbs, but, 

 to compensate, they have fleshy roots (Isochilus). Attention has 

 already been directed to the orchid Stenoptera which has no water 

 reservoir, as it grows in damp, gloomy spots. 



The most curious structural peculiarity which is common to all 

 epiphytic orchids (almost without exception) is the velamen possessed 

 by the roots. Typically this envelope is made up of a number of 

 layers of tracheides with perforated walls. These absorb water 

 deposited as dew or flowing down the surface of the host ; but it is 

 quite a mistake to suppose that they have any appreciable power of 

 condensing moisture from the atmosphere. The velamen is not always 

 an absorptive layer. In Bromheadia alticola (6) it is a sheath to prevent 

 the evaporation of water which is absorbed by root-hairs which attach 

 the plant to the substratum. 



The question arises, "Is the velamen really adaptive?" I 

 am of opinion that the answer must be given in the affirmative, in 

 spite of the cautious scepticism of Schimper on the subject. With 

 the single exception of one species of Stenoptem, all epiphytic orchids 

 have a velamen. All other known species of Stenoptera are terrestrial, 

 and this species only grows in damp, moss-laden, furrowed bark in the 

 lower regions of forests ; moreover, its structure is purely terrestrial. 

 We must conclude that this Stenoptera has only recently adopted its 

 epiphytic mode of existence (6). On the other hand, with the exception 

 of two species, all terrestrial orchids known are devoid of a velamen. 

 Epidendrum cinnabarinum is one of these exceptions, and has a velamen. 

 Now it is almost always a terrestrial orchid, but Fritz Muller 1 has found 

 it as an Epiphyte. It is a renegade Epiphyte which has become terres- 

 trial, as is confirmed by the occurrence on it of useless aerial roots, and 

 by the fact that its relatives are Epiphytes. The other exception is the 

 purely terrestrial Bromheadia palustris (6). It undoubtedly has been 

 derived from an epiphytic form. It possesses a curious impermeable 

 velamen like that of the epiphytic B. alticola ; but this velamen is exceed- 



1 Communicated by letter to Professor Schimper. 



N 



