THE AMERICAN BOTANIST. 
The roots of epiphytal Orchids differ from those of terrestrial 
plants is being covered with a white papery substance, called the 
velamen, and the cells of the velamen serve the double purpose of 
condensing or absorbing aqueous vapour from the atmosphere, 
when any is present, and of protecting the underlying cells from 
excessive evaporation during periods of drought. In its absorp- 
tive character it is analagous with the outer cells of the bog-mosses 
{sphagnum) and Leiicohryum, where the small chlorophyll-bear- 
ing cells are covered by large colourless cells, having very thin 
walls, which are variously perforated, and not only absorb water 
in liquid state, like a sponge, but also have the power of condens- 
ing 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 w^ith 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. Alany of these aerial roots do not become attach- 
ed 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 w^hich are furnished with aerial roots perish if planted in 
soil in the ordinary w^ay, but a few of them will occasionally bury 
their roots spontaneously in the earth, in which case they cease 
