INFLUENCE OF ENVIRONMENT 281 



ucijtter, to a certain extent the free nitrogen of the 

 or tosphere, which otherwise is quite unavailable for 

 desti.food. 



the hi ally, there is always found in connection with the 

 reduf of many trees, especially the Cupuliferse (oaks, 

 into les, etc.), certain fungus filaments, or "mycor- 

 latfia," which appear to take the place of root-hairs, and 

 While parasitic to some extent upon the roots, neverthe- 

 less are of great importance to their host in supplying 

 it with food from the soil. 



o. 



Pbotection against Cold 

 m 



m(550 far as can be judged from the geological" evidence, 

 are temperature of the earth was formerly more uniform 

 cases, 't present, and consequently the flora was also 

 are iruniform and composed of types vsrhich now belong 

 whici' temperate or sub-tropical zones. It is likely that 

 deriveapart of these plants were evergreen, as is now 

 curious in the warmer parts of the world. As the cli- 

 relatigrew more severe with the oncoming of the glacial 

 Siepn, it is probable that the deciduous habit was de- 

 Inioped in response to this, the only evergreen trees of 

 dagh latitudes at present being the Conifers, most of 

 evhich have retained their primitive evergreen habit. 



Where there is each year a long period of cold 

 weather, during which growth ceases entirely, it is 

 clear that trees with broad leaves, exposed to the severe 

 cold, and to loss of water by evaporation, are at a great 

 disadvantage compared to those which shed their leaves 

 at the end of the growing period and whose dormant 

 buds are thoroughly protected by the thick scales de- 

 veloped about the winter buds of all deciduous woody 



