176 NATURAL SCIENCE. Sept., 



Class II. — Epiphytes which absorb the nutritive solutions by means of their 



leaves. 

 Class I. — Group i. 



The first group of Epiphytes is made up of forms which merely 

 absorb the nutritive solutions occurring on the surface of the host- 

 plant. 



The simplest of these do not differ in any way from terrestrial 

 plants. They live in the gloomy depths of the forest on the mossy or 

 furrowed bark of the lower parts of trees (Hy menophyllacese, Lycopodium , 

 and an orchid Stenoptera). 



However, most adopt measures to guard against injury from 

 excessive loss of water. A few forms evade injury in this respect 

 simply by their ability to endure being completely dried up without 

 being harmed. This is well known in the case of lichens and mosses ; 

 but certain tropical polypodies are endowed with similar vitality. 

 Rainless weeks of the blazing West Indian sun are insufficient to 

 scorch out of existence Polypodium incanum, parched though it be. 



In the majority of genera there are distinct structural modifi- 

 cations in the form of water-reservoirs. Parts of them become fleshy 

 and store up water. Fleshy leaves occur in ferns, orchids, and 

 Gesneraceae ; bulbs in Amaryllidaceae ; tubers in ferns, and epiphytic 

 Utricularias ; fleshy roots in Vacciniaceae, and the curious mela- 

 stomaceous Pachycentvia. 



This water may be stored up in intercellular spaces, in ordinary 

 parenchymatous cells, or in specialised cells ("aqueous tissue," or 

 " storage tracheides "). 



The leaves of certain Gesneraceae have, between the upper 

 superficial layer and the green assimilatory tissue, aqueous tissue in 

 the form of colourless cells. This tissue increases in size late in the 

 life of the leaf, long after all other morphological changes have been 

 accomplished. Isolating old leaves for four weeks, they diminish in 

 thickness only to a very slight degree ; but if whole branches be 

 isolated for the same period, the old leaves become much thinner (and 

 gradually die), because the aqueous tissue has parted with water, and 

 has shrunk ; but the young leaves are scarcely any thinner than at 

 the commencement. These observations show that the aqueous tissue 

 is a reservoir on which the younger leaves can draw, and that it takes 

 on its peculiar function late in the life of the leaf. 



It is, however, among aroids and orchids that the most interesting 

 arrangements occur. 



In the Araceae, aqueous tissue is not known. Philodendron cannijolium 

 may be chosen as a type of aroid belonging to this group of Epiphytes. 

 It is a large herb with its leaves forming a rosette more than three 

 feet in height. The stem is short and gives off numerous roots. 

 Each leaf has a tongue-like lamina and a spindle-like petiole. The 

 leaf possesses spongy tissue with large and conspicuous intercellular 

 spaces. In the dry season the spaces are occupied by air, but when 



