102 GENERAL REVIEW OP THE ORCHTDE.E. ' 



On the mountain ranges of great altitude as the Himalaya in the 



eastern and the Andes of Colombia and Peru in the western section, ' 



the vertical range of orchids is considerable. On the Andes they ascend to ' 



elevations where the average annual temperature is less than in the j 



lower and median latitudes of the temperate zone, some even ascending \ 



to the immediate vicinity of perpetual snow as Epidendrum f.igidum, \ 



Onddium cucuUatum var. niihigenum, Odontoglossnm dengijfottim, and j 



others. It must not, however, be inferred that such orchids are hardy, I 



that is to say — that they can be cultivated in the open air in this i 



or any other country equally remote from tlie equator. Those epiphytal I 



orchids that occur at the highest altitudes, as the species just mentioned, j 



some of the Odontoglots of Colombia, the ]Masdevallias and Epidendra | 



of Peru, etc., are never subjected to such extremes of temperature as ! 



is sometimes experienced in the south of France, in Italy, and the I 



middle and southern States of Xorth America, and under which they j 



would perish. Apart from physical obstacles as the Himalaya in Asia, ! 



the Sahara in Africa, the arid tracts of northern Mexico and the ' 



Pampas of Argentina which prevent the spread of such orchids beyond j 



their present sphere, climate alone would prove fatal to them. I 



But little explanation is needed of the accompanying maps which [ 



have been prepared especially for this work by Mr. Edward Stanford, I 



of Charing Cross. The red lines called isothermal lines, or simply I 



isotherms, mark the limits of areas of equal temperatures expressed 



in degrees Centigrade for reasons to be presently stated; these are , 



the annual mean temperatures at sea-level. The annual mean \ 



temperature of places above sea-level, especially on mountain ranges, j 



may be approximately found by deducting one degree Centigrade ; 



for every 600 feet of altitude from the given mean. The gradations 



of blue colour indicate the annual average rainfall according to the 



references on the maps. j 



The following considerations should have weight in favour of the 

 adoption of the Centigrade thermometric scale in horticultural i 

 practice in the place of the Fahrenheit scale. 



The temperature of crystallisation or the freezing point as it is familiarly 

 called, if the water be kept imder the same pressure is constant ; the j 

 normal pressm'e of the atmosphere at sea-level when the barometric column ' 

 is 30 inches is about fifteen pounds to the square inch. The temperature 

 of condensation from the state of steam, in common parlance the boiling 

 point, is also constant as long as the pressure remains the same. There are , 

 thence two invariable standard points of temperature. On the theimometric 

 scale the space between the freezing and boiling points was divided by 

 Celsius into 100 equal parts whence this scale has obtained the name of 



