THE AMERICAN BOTANIST. 
13 
to develop the papery envelope, when they exercise the same func- 
tions 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 
zo'ue behind the growing point, which are packed so closely togeth- 
er as to have a velvety appearance. These aerial roots never reach 
the ground or adhere to any substratum, and their function is en- 
tirely to condense and absorb the aqueous vapour from the air in 
which they are freely suspended, as has been proved by direct ex- 
periment. Plants which have this kind of root grow in places 
where the air is very moist all the year round, and where the tem- 
perature does not fall below the freezing point. Where the air 
becomes periodically dry they cannot exist, because they have no 
means of preventing excessive dessication, a contingency we have 
just seen to be provided for in the papery covering of the roots of 
epiphytal Orchids. 
In many parts of the tropics, although the rainfall is heavy, there 
is generally an intermission of several weeks, and it is obvious that 
the epiphytes would wither and die if some provision were not 
made against such a contingency. The Bromeliads have reduced 
the evaporation from their leaves to a minimum, and made them 
hard and tough. Epiphytal ferns store up moisture in thick, leath- 
ery, and scaly rhizomes. And among Orchids we find the modifi- 
ed stem known as the pseudobulb a unique contrivance for retain- 
ing leaves through the longest drought. In some cases when the 
leaves are thin, they fall away altogether, and the plant is reduced 
to a bundle of green pseudobulbs, attached to- its support by a few 
almost dry aerial roots, and thus it remains dormant until the re- 
turn of the rainy season. The deciduous Dendrohiums form very 
good examples of this type. Sometimes the pseudobulbs are large 
and very fleshy, and the leaves are not deciduous, as in the case of 
Oncidiums, Odontoglossums, etc., while some seem to rely almost 
entirely upon their thick leathery leaves as a means of storage, as 
in the case of Oncidiums of the Lanceamim and Jonesiamnn 
groups. Some there are which have both thickened pseudobulbs 
and thick leathery leaves, as certain Cattlcyas, etc. Conversely 
we find others which have nO' pseudobulbs, though the leaves are 
