SEPTEMBER, I912.] THE ORCHID REVIEW. 261 
Epidendrum, and various others which were mentioned in reviewing the 
different groups. 
It is in the vegetative characters, and especially in the assumption of an 
epiphytic habit, that the most striking development appears in the Epiden- 
drez. Epiphytes were derived from terrestrial forest plants, and the key to 
their evolution is found in the perpetual struggle to reach the light with- 
out the expenditure of the material necessary to raise a terrestrial plant 
to an adequately lighted spot, and in the successful adoption of a xerophilous 
habit. The least modified types of the Epidendree are still terrestrial ; 
others live in moist shady crevices of bark low down on the tree trunks, 
while the more specialised ones live in drier but better lighted situations 
higher up the trees, or sometimes on rocks or other places where suitable 
conditions are found. The presence of epiphytes indicates the existence of 
a warm moist climate, or at least of an adequate supply of moisture during 
the season of active growth, for numerous devices have been adopted to 
carry them successfully through periods of drought. 
One of the earliest stages in the adoption of an epiphytic habit is seen 
when the subterranean or prostrate stem of a terrestrial Orchid begins to 
ascend the tree trunks, where a plentiful supply of moisture or accumula- 
tions of decaying vegetable matter rendered the change possible, and this was 
quickly followed by the production of aérial roots, with a modification of the 
root-bark into a grey protective covering, formed of air cells, and known as 
velamen. Then came the production of lateral branches—in this case 
known as secondary stems—bearing the leaves and becoming variously 
thickened into fleshy pseudobulbs, forming store-houses for water and other 
nutritive substances, to enable them to tide over periods of drought, when 
vegetative activity is more or less suspended. In some cases these secondary 
stems are scarcely thickened at all, as in Pleurothallis; in others they are 
so excessively short as to appear absent, as in Masdevallia, or the amount 
of thickening may vary enormously in detail, and may affect one or several 
adjacent internodes, as may be seen on comparing such well-known and 
diverse types as Dendrobium, Bulbophyllum and Cattleya. Or they may 
become hollowed, forming receptacles for ants, as in Diacrium and Schom- 
burgkia, the presence of the ants keeping cockroaches and other predacious 
insects in check, the plants thus providing a home for the ants in return for 
very effective services rendered. 
Modifications of foliar structure are seen in the attcnlated leaves of many 
genera, an adaptation securing the fall of the leaf, when no longer of service, 
and when its presence might be injurious by preventing the access of light. 
The leaves may also become fleshy, affording a storehouse for water, as in 
the Pleurothallis and Cattleya groups. 
Some of the characters which we have just considered reach a still 
