20 .MASDKVAl.l.lA 



from the latitude of Popayan northwards to Antioquia, the best 

 known saccolabiate Masdevallias occur general]_y at a lower elevation 

 than their cono-eners on the central and eastern rauo:es. North of 

 the isthmus the species are more scattered. Several interesting 

 forms occur in Costa Rica, thence northwards they become more 

 rare, till within the Mexican territory they entirely disappear. The 

 Masdevallias attain their highest vertical range near their southern 

 limit in Peru, where they ascend to between 9,500 and 13,000 feet. 

 In New Grranada they occur at a lower elevation, their vertical 

 range extending from 6,000 to 11,000 feet, and perhaps occasionally 

 higher, some small-flowered species ascending to near the snow-line; 

 near Sonson, which may be regarded as a Masdevabia '' centre/' 

 their altitude is from 6,000 to 7,500 feet. North of the isthmus, 

 as far as the Mexican frontier, the mountains and table-lands have 

 a lower average elevation, and the vertical range of the Masdevallias 

 is accordingly lower than in South America.* 



In these elevated regions the Masdevallias live under climatic 

 conditions different from those we experience in Great Britain, and 

 w^hich cannot be but imperfectly imitated in the glass structures in 

 which they are cultivated in this country ; nevertheless their culture 

 is not attended with any special difficulties. The climate peculiar to 

 their habitat is none the less eminently deserving of the attention of 

 cultivators, the chief phenomena of w-hich may thus be summarised : — 

 At the great altitude at which the Masdevallias arc found, especially 

 towards their southern limit, the pressure of the atmosphere is so 

 much diminished, that the air is not only much rarer but it is also 

 much colder, and its capacity for absorbing moisture is also greatly 

 diminished, Init owing to local causes the saturation point is constantly 

 being reached.! Now the average pressure of the atmosphere at sea- 



* The region here sketched, and the localities mentioned, are contained in the maps 

 illustiating the distribution of Odontoglossum and Cattleya, thence rendering the prepara- 

 tion of a special map for Masdevallia unnecessaiy. 



t Atmospheric saturation at the altitude at which the Peruvian Masdevallias live, and in a 

 somewhat less degree those of New Granada and the other localities named above, signifies 

 something different from what it does at the proximity to sea-level at which they are cultivated 

 in England. The quantity of moisture contained in a given quantity of air at an elevation of 

 1"2,000 feet is but a small part of what the same quantity of air is capable of containing at sea- 

 level in the same latitude. Thus, at or near sea-level, "one cubic metre of air saturated with 

 moisture at 25° C. (77° F. ) contains 22 '5 grammes of water, and if the temperature of the air 

 be reduced to 0° C. (32' F.), it will then be capable of retaining only 5 '4 grammes of water." 

 (Roscoe, Elcmeniary Chemistry, p. 51.) Or to express tliese facts in more popular language, a 

 cubic yard of air at sea-level and at the temperature of 77° F. is capable of containing about 

 five-.sixths of an ounce of aqueous vapour, but the same quantity of air at the freezing 

 point can only contain about one-fifth of an ounce. Hence at an altitude of 9,000—12,000 feet, 

 where the density of the atmosphere is reduced to nearly one-half, the actual quantity of 

 moisture held in suspension at any temperature is considerably less than at sea-level, 



