222 ABSORPTION OF WATER BY EPIPHYTES. 
freely in the air. They are not infrequently to be seen crowded together in great 
numbers at the base of the plant, forming regular tassels suspended from the dark 
bark of the branches as may be seen in fig. 50, where an Oncidiwm is represented. 
Each of these aérial roots is invested externally by a white membranous or 
papery envelope, and it is the cells of this covering that own the resemblance, above 
referred to, to the cells of Lewcobrywm and Bog-mosses. Their walls are furnished 
with narrow, projecting spiral thickenings and therefore do not collapse, notwith- 
standing their delicacy or the circumstance of their inclosing at times an air-filled 
cavity; they are further abundantly perforated, two kinds of apertures indeed 
being found. The one variety arises in consequence of the tearing of the portions 
of the cell-wall situated between the rib-like projections and consisting of extremely 
thin and delicate membranes (see fig. 49*); the existence of the other variety is due 
to the detachment of the cells which protrude in the form of papille, the result 
being, in this latter case, the formation of circular holes very similar to those 
already described as occurring in Leucobrywm. The cells resembling papillae have 
the peculiarity that they roll off when they get old in the form of spiral bands. 
The holes, of course, can only occur on the external walls of the outermost cells 
which border upon the open air, whilst in the interior the communication between 
the cells themselves is established by means of the rents previously referred to. 
The entire covering thus composed of perforated cells may be compared to an 
ordinary sponge, and, indeed, acts after the manner of a sponge. When it comes 
into contact with water in the liquid state, or more especially when it is moistened 
by atmospheric deposits, it imbibes instantaneously its fill of water. The deeper- 
lying living green cells of the root are then surrounded by a fluid envelope and are 
able to obtain from it as much water as they require. 
But these roots also possess the power of condensing the aqueous vapour 
contained in the air. They act upon the moist air in which they are immersed in 
exactly the same way as spongy platinum or any other porous body. If the aérial 
roots of Oneidium sphacelatum are transferred from a chamber full of dry air to 
one full of moist air, they take up in 24 hours somewhat more than 8 per cent of 
their weight of water, those of Epidendron elongatum absorb 11 per cent, whilst in 
the case of many other tropical orchids the amount thus imbibed is doubtless much 
more considerable still. 
The power of condensing aqueous vapour, and other gases as well, is of the 
greatest importance to these plants. The tree-bark serving as their substratum, to 
which they are fastened merely by a few fibres, is anything but a permanent 
source of water. Such water as the bark does contain reaches it, not from the 
interior of the trunk and indirectly from the soil in which the trunk has its roots, 
but from the atmosphere; that is to say, from the very source whence the 
epiphytes upon the bark must also derive their supply. Now, when on the occa- 
sion of a long-enduring uniform aérial temperature, there is a failure of atmos- 
pherie deposits, which is a regularly recurring circumstance in the habitat of the 
orchids in question, the sole source of water left is the vapour in the air, and the 
