BIONOMIC CHARACTERS OF PLANTS 1005 



All degrees of freshwater phanerogams are found, from those 

 having only their roots in the water, as many of the larger swamp 

 plants, to those nearly or entirely submerged. Parasitic, saprophytic, 

 and epiphytic phanerogams are further characteristic types adapted 

 to peculiar habitats. In short, the variety- and adaptability of the 

 spermatophytes are as multitudinous as the variation in the character 

 of the terrestrial realm. 



The coniferous gymnosperms are almost wholly confined to 

 temperate climates, especially the colder belts, but cycads are most 

 characteristically tropical plants. Both monocotyledons and dicoty- 

 ledons are more commonly represented by trees in the tropical than 

 in the temperate zone, the number of trees diminishing toward the 

 colder belts, where the conifers increase in number. 



Arboreal vegetation is characteristically unknown in regions 

 where the sub-soil is permanently frozen, i. e., where the mean sum- 

 mer temperature is below 10° C. Such is the character of the tree- 

 less plains of northern Canada, the coldest part of the North Ameri- 

 can continent, where the mean annual temperature is below — 8° C. 

 and the mean summer temperature below 10° C. Here sedges, 

 grasses, and lichens predominate, while trees and sphagnum-bogs 

 are conspicuously absent. 



The present northern extent of the forest regions of Canada 

 is limited by the mean summer temperatures of io°-i5° C. Here 

 the poplar (Populus tremuloides and P. balsamifera), the birch 

 (Betula alba), spruce {Picca alba and P. nigra), pine (Pinus baiik- 

 siaiia) , and larch {Lari.v amcricana) occur, while beneath them 

 the ground is often an extensive sphagnum swamp. "Poplar, birch, 

 and pine extend northward as far as the heavy forest extends, 

 while larch and the true species of spruce extend northward to 

 the northern limit of trees, becoming small and dwarfed before 

 they finally disappear." ( Tyrrell-28 i^cVp-pz.) 



South and west of the forested area are the grassy plains, which, 

 because of their dryness, do not support trees or sphagnum bogs. 

 As these plains were in the condition of the frozen tundra after 

 the retreat of the ice, and with the amelioration of the climate 

 became dry, no trees or sphagnum bogs ever developed there. 



Ecology and Ecological Adaptations of Sphcrmatophytcs. 



]\Iodern spermatophytes, as a whole, are divisible into a number 

 of habitudinal types, among which the following are the most 

 important : 



