ECOLOGICAL 157 



composition; and there is sometimes a deficiency in salts, such as 

 compounds of nitrogen, phosphorus, potassium, and calcium. But 

 different kinds of plants differ not a little in their requirements, and 

 thus one of the consequences of deficiency in the soil may be that 

 certain species are excluded, while others are favoured. Thus bog- 

 mosses (Sphagnum) flourish best where there is little lime ; and there 

 are interesting cases, as in Gentiana excisa and G. acaulis, of what 

 looks like the splitting of a species into those that cannot stand 

 much lime and those that can. 



Just as members of the same species may become large or may 

 remain minute according to the soil (and other environmental 

 factors), this being a matter of individual modification, so there are 

 giant and dwarf varieties of presumed germinal origin which are 

 adapted to luxuriant and niggardly conditions. Thus in the Barren 

 Grounds or Tundra of the Far North, where the soil is often extremely 

 poor, and where other conditions, such as the long winter, are 

 adverse, there are many constitutional dwarfs, such as birch-trees 

 and willow-trees only a few inches in height. Another adaptive 

 characteristic of plants frequenting poor or unready soil is the 

 lengthening of the roots, rootlets, and root-hairs. Neger notes that 

 plants so adapted to make the most of soil that is deficient in certain 

 constituents, nitrogen-compounds in particular, are readily killed 

 if transplanted into a rich garden. The sundew is said to be killed 

 if supplied with "hard" water. 



In bog-mosses, which are also very sensitive to more than a 

 minimal quantity of salts, there is a large capillary system between 

 and in the leaves, so that, as everyone knows, they are like sponges 

 in the quantity of water that they can retain. Moreover, the colloidal 

 character of the cell- walls facilitates absorption. The nitrogenous 

 food-materials are chiefly obtained from the rain and the included 

 dust-particles. The same seems to be true of the leaves of the rootless 

 epiphytic Tillandsias, sometimes called "vegetable horsehair", 

 which are abundant on the branches of trees in South American 

 tropical forests. The upper surface of the leaf is in some species 

 thickly covered with peculiar flattened scales or hairs, which absorb 

 the rain-water or the dew and its dissolved particles. There are 

 absorptive hairs on some of the Orchids of the Far East which live 

 high above the ground, and there are Javanese mosses which hang 

 from the leaves of forest trees and absorb water by their whole 

 leaf-surface. In another section we shall refer to cases where scarcity 

 of food-material in the soil is circumvented by partnership with a 

 fungus, as in some heaths, or with symbiotic bacteria, which fix the 

 atmospheric nitrogen, as in all Leguminous plants. 



INSECTIVOROUS PLANTS.— The majority of insectivorous (or 

 more strictly carnivorous) plants occur on moorland soil, which is 



