GEOGRAPHICAL DISTRIBUTION. 6 



and to the cultivator, will afford an idea of the 

 requisite temperature whenever the latitude and 

 elevation is stated with newly-introduced plants. 



Although, in the present instance, we have only 

 to treat of tropical plants, and therefore need not 

 extend our remarks beyond the latitudes in which 

 they are found; it may be well to mention another 

 important matter connected with their geogra- 

 phical distribution, affecting their cultivation 

 most materially ; this is the difference which ex- 

 ists in the state of the atmosphere at different 

 altitudes. The effect of elevation is to cause a 

 reduction of temperature and rarefaction of the 

 surrounding air, the latter produces an increase 

 in the intensity of light, and, of necessity, a cor- 

 responding decrease of humidity ; now, though 

 Orchidaceous plants inhabit only those places 

 where moisture is prevalent, at least for a season, 

 and consequently are seldom found at very 

 considerable elevations, still a degree of differ- 

 ence is observable among them of sufficient ex- 

 tent to render attention to these circumstances 

 absolute and indispensable. I have therefore 

 thought it advisable thus briefly to point to it 

 here, leaving the application to be explained when 

 treating of ventilation and the benefit of two 

 departments for their culture. 



Generally speaking, the more elevated the sta- 

 tion of the plant is, in its natural localities, its 



