INFLUENCE OF ENVIRONMENT 281 



matter, to a certain extent the free nitrogen of the 

 atmosphere, which otherwise is quite unavailable for 

 plant food. 



Finally, there is always found in connection with the 

 roots of many trees, especially the Cupuliferse (oaks, 

 beeches, etc.), certain fungus filaments, or "mycor- 

 rhiza," 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. 



PROTECTION AGAINST COLD 



So far as can be judged from the geological evidence, 

 the temperature of the earth was formerly more uniform 

 than at present, and consequently the flora was also 

 more uniform and composed of typej which now belong 

 to the temperate or sub-tropical zones. It is likely that 

 a large part of these plants were evergreen, as is now 

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

 mate grew more severe with the oncoming of the glacial 

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

 veloped in response to this, the only evergreen trees of 

 high latitudes at present being the Conifers, most of 

 which 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 



