PERIODICAL LITERATURE 



BOTANY AND ZOOLOGY 



The preponderance of spruce in north-central 

 Root Habits Canada is usually ascribed to its greater tolerance 

 of Trees in of* low temperatures than that of the predominat- 



Northern Canada ing trees of the more temperate climates. It is 

 frequently inferred that the direct effect of tem- 

 perature upon physiological processes controls plant distribution in the 

 far north. Howard E. Pulling shows how low temperatures may re- 

 tard the growth or limit the size of certain arborescent species in an 

 indirect way. 



The root habits of Picca mariana, Pimis banksiana, Larix laricina, 

 Betitia papyrifera, Populiis balsamifera, and Pinus strobus were studied 

 in the province of Manitoba between latitude 55° N. and 56° N. and 

 longitude 96° W. and 98° W. in a uniform clay soil and in a sandy 

 soil, near the south shore of Lake Superior, in Douglas County, Wis. 

 The main characteristics of the root systems are exhibited in dimen- 

 sioned figures. The soil was found to be generally shallow and frozen 

 at depths ranging from 2 meters on the exposed south slopes to 3 cm. 

 on flat benches, with a northern exposure which acted as a mechanical 

 barrier to root penetration. 



These trees were found to dififer not only in their root habits, as they 

 do in their top habits, but also in the rigidity with which the habits are 

 maintained under varying environmental conditions. This investiga- 

 tion is summarized as follows : 



Root systems may be classified as deep when the habit is centered 

 about a main deeply penetrating tap root and shallow when such a tap 

 root is absent, and the roots remain near the surface of the soil. Various 

 degrees of transition may be recognized, but the important point is that 

 some trees have a very rigid root habit, while with others it is more 

 flexibe. Deep root systems of an inflexible nature cannot produce 

 large trees in shallow soils, whether the shallowness is caused by rock 

 or ice. Trees whose root systems are flexible and are not too deep 

 rooted in deep soil may endure shallow soils. The degree of flexibility 

 of habit and the degree of penetration in deep soils may determine the 

 northward distribution of many plants, regardless of relations between 

 the plant and its environment that may exclude other species from those 



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