36 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



heat (a mean temperature of at least 20 C.) and water ; their present home hes 

 particularly in moist tropical tracts, but at an earlier date they were certainly 

 widely distributed. 



2. Xerophilous : Plants caUing for much heat, but making the most modest 

 demands for water. Here belong plants of the desert, steppe, and savannah. 



3. Mesothermic : Plants calfing for an annual mean temperature of i5-20 C, 

 and, at least during certain periods, abundant moisture. In the Tertiary epocli 

 these extended up to the North Polar lands. 



4. Microthermic : Plants requiring an annual mean temperature of 0-15 C, 

 little of the sun's heat, uniformly distributed atmospheric precipitations, and a 

 period of rest caused by the cold. 



5. Hekistothermic : Plants living beyond the Umits of tree-growth, where the 

 annual mean temperature sinks below C. ; they endure prolonged lack of Ught. 



6. Megistothermic plants existed in earher ages of the world's history, and 

 demanded high uniform temperatures (above 30 C). They were especially Crypto- 

 gamia. 



A. de Candolle's groups suffer from the defect that no plants are dependent 

 upon the mean annual temperature, but that plants depend upon the duration 

 and temperature of the vegetative season, and upon certain minima of temperature 

 and humidity which must not be transgressed. As an emendation more consonant 

 with nature, the following arrangement is proposed.' 



1. Hydromegathermic plants: mean temperature of the coldest month being 

 more than 16 C. 



2. Xerophilous plants : the rainiest month having less than twelve days with 

 rain. 



3. Mesothermic plants : mean temperature of the coldest month being below 

 16 C. yet not below C. for long together. 



4. Microthermic plants : winter having periods of prolonged frost (with snow 

 remaining on the ground). 



5. Hekistothermic plants : mean temperature of the warmest month being less 

 than 10 C. 



CHAPTER VIII. MOVEMENTS OF THE AIR 



Wind exerts an influence upon both the configuration and the distri- 

 bution of plants. This is to be seen most clearly when it blows over 

 large stretches where its force is not broken by mountains, forest, or town, 

 as is the case on sea-coasts and on extensive plains, such as the Asiatic 

 steppes, the Sahara, and the like ; it is also seen where a definite wind, 

 the trade- wind, prevails. 



The effects are revealed on tracts with a loose sandy soil and a 

 scanty covering of plants, for instance, on many coasts and the Sahara, 

 in the formation of dunes, with which is associated a highly characteristic 

 vegetation. 



They are furthermore revealed in mountain-districts by the higher 

 atmospheric humidity and greater rainfall, which are caused by the daily 

 valley-winds.2 Likewise on lofty mountain-chains they are reflected in 

 the distribution of the atmospheric precipitations, because the windward 

 side catches the moisture brought by the wind (e. g. the east and south- 

 east coasts of Australia, the east side of the Andes), while the lee-side 

 remains dry. Dependent on these circumstances is the distribution of 

 the different plant-communities according to the amount of moisture 

 that they require, many species and whole formations being restricted 

 as to their altitude above sea-level, and as to their environmental bounds. 



' See Koppen, 1900. " Hann, 1897, 1901 ; Vahl, 19046; Scott-Elliot, 1900. 



