238 FERNOW 



tected against variations of atmospheric conditions, give 

 an advantage to these forms of life in northern regions 

 which is lacking in the arborescent flora, with its persistent 

 growth, its long period of life before maturity is reached, 

 and elevation above the ground. 



The wintry blasts which are of no moment to the her- 

 baceous plants and shrubs, must be endured by the arbor- 

 escent flora ; and late frosts in the spring, which may find 

 the former in condition for withstanding their blight, will 

 nip the tree buds which an early warm spring sun has 

 called into premature activity. 



Again, while the herbaceous plant readily survives an 

 extraordinarily unfavorable season, and with its prolific 

 annual seed production soon recovers lost ground, the tree 

 individual, after having weathered many winters, may fall a 

 prey to a single exceptional season; moreover, seed pro- 

 duction in the tree, coming only late in life and at longer 

 or shorter intervals, is less favorable to reestablishment. 



Again, while the low vegetation is able to subsist on a 

 modicum of soil, the tree, as it grows in height, requires 

 corresponding root space, both to supply itself with water 

 and to brace itself against the winds the leverage 

 increasing with the growth. Soil conditions in the 

 competition between different forms of vegetation may 

 become so important that climatic conditions are of 

 secondary moment; thus we find grass and weeds suc- 

 cessfully keeping out the arborescent flora where no 

 climatic impediments to the latter exist. 



The combination of conditions influencing forest growth 

 is then, it must be admitted, more complex than that 

 which determines the distribution of smaller plants, and 

 hence not only does the composition of the forest vary 

 according to the adaptability of the species, but at the 

 same time the individual development and density of 

 stand vary with the different conditions. 



