73 



is given by Angot^, according to whose observations the harvest of winter r^'e 

 in France is delayed on an average about four days, as the elevation increases 

 about lOO meters. Attention should be called, however, to the circumstance 

 that, with increasing height, the air being thinner is less warm so that there- 

 fore it must have an appreciable effect on the development of vegetation. 

 With this should be reckoned conditions of moisture which, aside from the 

 physical constitution of the soil, are different for plants of Alpine regions in 

 lower latitudes than for those from plains in the Arctic zone. Within the 

 same degree of latitude mountains, as colder bodies, will condense more 

 water vapor and thereby bring about more abundant precipitation than takes 

 place on plains. On this account more snow will fall and the warmth needed 

 to melt this greater mass of snow is withdrawn from vegetation. Even after 

 the snow has melted in spring, the plants in the mountains will nevertheless at 

 first be less able to benefit from the sun's warmth than those on the plains 

 since the inequalities of the upper surface of the soil become efTective. A 

 square meter of very broken ground surface has a much greater upper sur- 

 face, divided into many slanting levels, over which the same amount of 

 warmth must be distributed, than has perfectly level land, the different par- 

 ticles of which are raised to a higher temperature. This is the case in moun- 

 tain chains in contrast to level plains. It is evident from these statements 

 that with increased elevation above the sea these processes of weathering and 

 decomposition must be retarded since they are essentially favored by 

 warmth. It is also evident that such peculiar combinations of vegetative 

 factors will produce characteristic forms, of which the best known feature 

 is short, repressed growth. Such forms of growth are kept constant, first of 

 all, in the seeds. Climatic forms which have become hereditary in this way 

 liave been termed "Oecological variations"'-. 



If it was said at first that the temperature of the air at higher levels is 

 lower, it must also be emphasized, on the other hand, that at higher levels the 

 intensity of the illumination increases and produces accordingly greater soil 

 zvarmth. On this account climate of the lower and middle latitudes, on ac- 

 count of the greater intensity of light and greater warmth of the soil, would 

 differ favorably from that of those plains in a Polar zone where the tem- 

 perature of the air is the same. The lesser atmospheric pressure in moun- 

 tains must result in an increase of transpiration as stated by FriedaP 

 and the increased supply of light in an increase of the assimilatory activity of 

 the leaf. Consequently the typical mountain plant works more energetically 

 and in this way is explained its shortened vegetative period. 



According to the observations of Bonnier*, who made experimental 

 gardens on Mt. Blanc and in the Pyrenees, in Alpine climates with a 



1 Der Naturforscher, 1883, No. 24. 



2 Lebensgeschichte der Bliitenptinnzen Mitteleuropas. Von Kirchner, Loew und 

 C. Schroter. Stuttgart, Ulmer 1904. p. 116. 



3 Friedal, Action de la pression totale sur I'assimilation chlorophyllienne. C. rend. 

 1901. Cit. Bot. Jahresb. 1901. Section II, p. 221. 



* Bonnier, Etude experimentale de I'influence du climat alpin sur la vegetation 

 etc. Bull. Soc. Bot. France. Vol. XXXV. 35. 1888. 



