THE ORCHID REVIEW. i 39 



state, like a sponge, but also have the power of condensing it when in the 

 form of vapour. A double function appertains to the roots of epiphytal 

 Orchids, firstly to fix the plant to the bark, and secondly to supply it with 

 nutriment. When the tip of a root comes in contact with a solid body it 

 adheres closely to it, flattens itself out more or less, develops papilliform or 

 tubular cells, which grow into organic union with the substratum, and act as 

 holdfasts. But if the root extends beyond the limit of the substratum it 

 ceases to develop these clamp-cells, and hangs down in the form of a white 

 filament, and many of these aerial roots do not become attached at all but 

 hang freely in the air, often forming regular tassels. 



The power of condensing aqueous vapour, and other gases as well, is 

 of the greatest importance to these plants, for the bark to which some of 

 them are attached is anything but a permanent source of moisture, and 

 when this supply fails the only possible method of acquiring it is by 

 condensing any that may be present in the atmosphere. When this supply 

 fails the velamen dries up, and then acts as a medium of protection from 

 excessive evaporation from the underlying tissue, and the plant then goes to 

 rest. It must not be supposed that epiphytal Orchids grow in a perpetually 

 moist atmosphere in the shady primeval forests. Many of them grow in 

 only partially shaded spots, and in regions where periods of drought occur 

 regularly every year, during which time there is a more or less complete 

 •cessation of vegetative activity, when the use of a protective covering for the 

 roots is at once apparent. Most Orchids which are furnished with aerial 

 roots perish if planted in soil in the ordinary way, but a few of them will 

 occasionally bury their roots spontaneously in the earth, in which case they 

 cease to develop the papery envelope, when they exercise the same functions 

 as in the case of land plants. 



Some other plants besides Orchids exhibit this papery covering on their 

 aerial roots, but more frequently these roots are furnished with a dense 

 fringe of so-called root hairs, arranged in a broad /one behind the growing 

 point, which are packed so closely together as to have a velvety appearance. 

 These aerial roots never reach the ground or adhere to any substratum, and 

 their function is entirely to condense and absorb the aqueous vapour from 

 the air in which they are freely suspended, as has been proved by direct 

 experiment. Plants which have this kind of root grow in places where the 

 air is very moist all the year round, and where the temperature does not 

 fall below the freezing point. Where the air becomes periodically dry they 

 cannot exist, because they have no means of preventing excessive dessica- 

 tion, a contingency we have just seen to be provided for in the papery 

 covering of the roots of epiphytal Orchids. 



We may now say something about the structure of epiphytes, and the 

 various contrivances made for preserving life during periods of drought. In 



