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 aérial 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 aérial 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 aérial 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 aérial 
root is placed in earth. Most orchids with aérial roots perish, it is true, when they 
are treated like land-plants and planted in soil; but a few species, on occasion, bury 
their aérial 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 
Aroidex exhibit the porous, papery covering on their aérial 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, which are surrounded by air; 
they are crowded very closely together and give the parts affected a velvety 
appearance. Besides several Aroidex, one of which (Philodendron Lindeni) is 
drawn on the left side of fig. 51, many other epiphytes, such as the South 
