PREFACE. Vll 



It is, however, the Plearothals that principally occur in such 

 elevated regions : Masdevallia, Restrepia, Stelis, and Pleuro- 

 thallis itself, constituting the most striking feature of the 

 Orchideous flora until the mean temperature rises to 56°, where 

 they begin to disappear. 



The genus Odontoglossum appears to be more impatient of 

 heat than its nearly alUed genus Oncidium, for a species is 

 found where the mean temperature is only 49°, and where it 

 sometimes even freezes ; the others are scattered over the moun- 

 tain slopes till they reach the lowest limits of their order, where 

 the mean temperature rises to 75°, where it is never cooler than 

 55° nor hotter than 80° ; in such a cHmate not one of the race 

 is found except a Schomburgkia, a Burlingtonia, an Odontoglos- 

 sum, and an lonopsis. In the hot lands on a level with the sea 

 Orchids do not seem to be able to exist. 



It is evident indeed that these Colombian species have no 

 affection for a high temperature, and that many prefer a low 

 one. No fewer than 13 occur between 10,000 and 11,000 feet, 

 where Humboldt tells us it is as cold as the mean of the month 

 of March near Paris j 19 where the mean is that of May in 

 the same city ; while the mean temperature of the zone between 

 5000 and 6000 feet, where the largest proportion exists, is only 

 that of Paris in August, according to the same authority. 



These, and many more interesting facts of a similar kind, 

 will strike every intelligent observer; they seem on the one 

 hand to show to the gardener the great importance of studying 

 well the chmate of the plants he cultivates, and on the other 

 they ought to teach the collector the incompleteness of any 

 memoranda in which the details of climate are not fully given. 

 General observations are not sufficient; particulars are indis- 

 pensable. Suppose, for example, on the assertion of some 

 generalising traveller, we were led to believe that the Masdevallias 



