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 Oncidium is represented. 



Each of these aerial 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 Leucobryum 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 3 ); the existence of the other variety is due 

 to the detachment of the cells which protrude in the form of papillae, the result 

 being, in this latter case, the formation of circular holes very similar to those 

 already described as occurring in Leucobryum. 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 aerial 

 roots of Oncidium 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 Upidendron 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 aerial temperature, there is a failure of atmos- 

 pheric 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 



