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 Cupulifere (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 types 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 
