ABSORPTION OF WATER BY EPIPHYTES. 223 



only possible method of acquiring that vapour is the condensation of it by the 

 porous tissue investing the roots. In the event of the air around the orchid-plant 

 containing temporarily but very little moisture, the porous tissue dries up, it is true, 

 very quickly; its cells fill with air and their function as condensers is interrupted. 

 But these air-filled cellular layers then form a medium of protection against 

 excessive evaporation from the deeper strata of the root's tissues, which might be 

 very dangerous in the case of this kind of epiphyte. There is a wide-spread 

 impression that the tropical orchids grow in a perpetually moist atmosphere in the 

 dark shade of primeval forests, and this preconception is fostered by pictures of 

 tropical orchids representing these plants as living in the most obscure depths of 

 woods. In reality, however, the orchids of the tropics are children of light. They 

 thrive best in sunny spots in open country. Those species in particular which have 

 their aerial roots invested each by a thick, white, papery, porous covering belong to 

 regions where a long period of drought occurs regularly every year, and where, in 

 consequence, vegetative activity is subject to periodical interruption, as it is in the 

 cold winter season of the more inclement zones. 



For epiphytes inhabiting these regions of the tropics a more expedient structure 

 of root cannot easily be imagined. In the dry season the papery covering reinforces 

 the safeguards against too profuse transpiration on the part of the living cells in the 

 interior of the root, and in the wet season it provides for the continuous supply of 

 the requisite quantity of water. In this sense the porous layer is to a certain 

 extent a substitute for wet soil, or, in other words, the concealment of the living 

 part of an aerial root in the saturated envelope is analogous to that of the root- 

 fibres of land-plants in the damp earth. The manner in which the water reaches 

 the inner cells of an aerial root from the saturated envelope is also quite 

 characteristic. Under the porous tissue lies a layer composed of two kinds of cells 

 of different sizes. The larger cells are elongated and have their external walls, 

 which are adjacent to the porous tissue, thickened and hardly permeable by water. 

 Between these lie smaller, thin- walled, succulent cells, which admit the water from 

 the porous envelope, and should therefore be regarded as absorption-cells. It is also 

 noteworthy that the porous, paper-like covering is discarded as soon as an aerial 

 root is placed in earth. Most orchids with aerial roots perish, it is true, when they 

 are treated like land-plants and planted in soil; but a few species, on occasion, bury 

 their aerial roots spontaneously in the earth and push off their envelopes, and then 

 the imbedded parts exercise the same functions as in the case of land-plants. 



We have already mentioned that, in addition to thousands of orchids, several 

 Aroidese exhibit the porous, papery covering on their aerial roots. But still more 

 frequently the air-roots of Aroids, which live as epiphytes upon trees, are furnished 

 with a dense fringe of so-called root-hairs in a broad zone behind the growing- 

 point. The hairs project on all sides from the roots, wdiich are surrounded by air; 

 they are crowded very closely together and give the parts affected a velvety 

 appearance. Besides several Aroideae, one of which (Philodendron Lindeni) is 

 drawn on the left side of fig. 51, many other epiphytes, such as the South 



