Marcu, 1916.] THE ORCHID REVIEW. 77 
g Z 
COLOGY has been defined as the science treating of the reciprocal 
relations of organisms and the external world. In less technical 
language, it is the science which ‘teaches us how plants or plant- 
communities adjust their forms and modes of behaviour to actually 
operating factors, such as the amounts of available water, heat, light, 
nutriment, and so forth.” (Warming, (Ecology of Plants, p. 2.) The 
ecology of Orchids is one of the most interesting aspects of the study of 
these intensely fascinating plants, since it furnishes the data primarily 
necessary to an intelligent understanding of their cultural requirements, 
based upon the physical conditions of their natural environment. The 
ecologic habit is often much more characteristically marked in some species 
than in others ; and it is the purpose of the present notes to deal briefly with a 
few of the former as representative types amongst the Orchids of Southern 
Mexico. 
Taking, as a first example, Oncidium Cebolleta, the peculiar structure of 
this Orchid, in the thick, fleshy, terete leaves, the nearly complete 
suppression of the pseudobulbs, and the smallness and scantiness of its 
thread-like roots, is at once significant of its adaptation to prolonged heat 
and drought. It is thus commonly found where such climatic conditions 
obtain, growing often, in forest, on the slender hold of lianas, or, in semi- 
open country subject to the influence of strong, dry, hot winds, on hard, 
thin-barked, thorny trees of a dwarf or scrubby nature. Its range in 
elevation is from sea level to one or two thousand feet above. In this same 
class, by reason of their ebulbous form and thick, fleshy leaves, may be 
included O. carthaginiense, O. luridum, and O. microchilum, although 
these frequently occur in more humid parts. 
Another Orchid very similar in habit is Brassavola nodosa, which also 
physiologically resembles the first-mentioned Oncidium, but has ampler 
roots, sometimes in a knotted bunch. B. nodosa rarely grows on high 
trees, which, indeed, seldom exist in extremely dry zones, lofty forest being 
generally incidental to the botanical evolution of very wet regions in the 
tropics. 
An Orchid which is one of the most distinctive, in respect to its ecology, 
of any found in Mexico is Brassavola Digbyana. This plant bears a very 
large and very beautiful flower, having greenish-yellow sepals, tinged on 
the outer side with rose-pink, yellowish-white petals of crystalline texture, 
similarly tinged and faintly veined, and elaborate silvery-white labellum, with 
fine gradations of pale green in the throat and with delicately frilled edges— 
NOTES ON THE ECOLOGY OF ORCHIDS. 
By J. L. HERMESSEN, F.R.G.S, 
