40 PLANT GEOGRAPHY 



mainly equatorial, but far more widely distributed in 

 early geological periods. 



2. Xerophilous, in climates in which no month has 

 twelve days with rain; including the " Succulent plants " 

 of gardens. 



3. Mesothermic, in a temperature in which the mean 

 of the coldest month is below 16 C., but which does not 

 fall below the freezing-point of water for long together; 

 requiring also, at least during certain periods, abundant 

 moisture: the "Greenhouse plants" of Sub -tropical 

 and Warm Temperate zones which require protection 

 from frost. These were represented within the Arctic 

 Circle in Tertiary times. 



4. Microthevmic, adapted to prolonged periods of frost, 

 requiring little heat, rain, or snow throughout the year, 

 and a period of rest caused by the cold : " Hardy plants " 

 in our climate. 



5. Hekistothermic, in a temperature in which the 

 mean of no month exceeds 10 C. (50 F.): "Alpine 

 plants " which live beyond the limit of tree-growth. 

 Accustomed to be covered for long periods by snow, 

 they can endure the long "night-" of Arctic latitudes, 

 but require protection from the full sunlight and cold 

 drought of an often snowless English winter. 



D. WIND 



Most plants can readily obtain all the oxygen they 

 require from the surrounding air. The slow diffusion of 

 air in water, especially in stagnant water, is associated 

 with large air-storing spaces and passages in aquatic 

 plants; while many plants growing in mud develop 

 special aerenchyma, or air-secreting tissue, mostly of 

 cortical origin, formed of thin-walled cells, and occa- 

 sionally special respiratory roots, or pneumatophores , 

 coated with such aerenchyma. The " knees " of the 

 Swamp Cypress (Taxodium distichum Rich.) are a 

 striking example. 



Wind, that is the air in movement, has great indirect 

 and direct effects upon plant-life and distribution. It 

 is the cause of wave-action, and when constant in 

 direction of ocean currents. It greatly modifies the 

 distribution of heat, both in such currents and in that 

 which it carries itself, and also that of atmospheric 



