1818.] M. Be Candolle on the Geography of Plants: 411 



the less difference there is between the summer and the winter ; 

 while the further we are from the equator, or from the level of 

 the sea, the more difference there is between the seasons. Hence 

 we observe an analogy between these two situations with respect 

 to climate, and a similarity in their vegetable productions. 

 Thus the tree which thrives the nearest to the pole is the birch, 

 and this same tree is found on the Alps above the region of the 

 pines. In general, evergreens are rare both in the northern 

 and much elevated regions, or, if they are found there, it 

 is only such as have awl-shaped leaves, on account of the snow, 

 which, accumulating on them, either freezes the young shoots 

 or breaks the branches. In the wild state, perennial plants are 

 more numerous in proportion to those that are annual, whether 

 we recede from the equator or from the level of the ocean. There 

 are certain causes which operate in a different manner upon 

 cultivated plants ; but this forms no objection to the general 

 principle which has been laid down, or to the analogy which we 

 observe between a northern latitude and a great elevation. 



It is well known that light acts powerfully upon vegetables. 

 It excites their transpiration, determines many of their motions, 

 and seems to produce in them many chemical changes. When 

 they are deprived of light they become white, flaccid, and 

 drawn out ; on the contrary, the effect of a bright light is to 

 render them more vigorous, and to increase their colour, firm- 

 ness, and all their sensible qualities. There are three different 

 ways in which plants are subjected to the increased action of 

 light, according to the situation in which they are found. In 

 the equatorial regions the sun acts upon them, during the whole 

 year, with great power ; in the northern countries, during the 

 summer, while the process of vegetation is going forwards, the 

 sun remains a long time above the horizon ; and in lofty moun- 

 tains, the rarity or purity of the air causes the light to be more 

 intense than in the plains. Many of the facts which have 

 generally been ascribed to the operation of temperature upon 

 plants would appear to be rather owing to the action of light. 

 M. Humboldt has particularly attended to this subject, and has 

 attributed to this cause the greater proportion of resinous and 

 aromatic substances, which is found in plants that grow 

 between the tropics, or on high mountains. It is probably owing, 

 in a great degree, to the less intensity of the light, that alpine 

 plants can seldom be made to grow luxuriantly on plains, where 

 they generally acquire a blanched appearance and a delicate 

 texture. 



In proportion as we ascend in the air, the hygrometer shows 

 that the degree of moisture continues to diminish. This habitual 

 dryness of the air of mountains tends to increase the insensible 

 perspiration of plants, and consequently the absorption by their 

 loots. The fluid necessary for this purpose is generally supplied 

 by the melting of the snow, with which the summits of the 



