RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM. 



115 



uudei'gi'ound stem, peculiarly desirable for absorption, is encountered, it is regularly 

 embraced by the suction cells, and as great an absorbent surface as possible is thus 

 brought into contact with the nutritious fragment. Indeed, tlie development of 

 suction cells on the roots of many gentians (viz. Gentiana ciliata, G. germanica, G. 

 Austriaca, and G. Rhcetica) is confined to the parts of the root-branches, which, in 

 the course of their passage through the vegetable mould, have come into contact 

 with a particularly nutritious portion of it. Wherever there is contact, the root is 

 thickened, and absorption cells project unilaterally from the eijiderrais and grow 

 into the decaying fragment of wood or bark which is to be drained of its nutrient 



Fig. 16.— Transverse section through absorption-roots of Saprophytes. 

 1 Gentiana Rhcetica. ' The Bird's Nest Orchis (Ncottia Nidus-avü). 



material (see fig. 16^). Roots of this kind remind one of the root-structures of 

 parasites which are furnished with so-called "haustoria", and which will be 

 discussed more in detail in subsequent pages. But they are different in that they 

 absorb food not from living but from decaying parts of the nutrient substratum. 



Most plants that grow on the vegetable mould of alpine meadows, and the black 

 earth deposited by snow-drifts in mountainous regions, develop flat instead of 

 tubular epidermal cells as suction cells, and in this resemble marsh-plants. In 

 many of these cases the roots are so abundantly and minutely ramified that they 

 form a plexus investing the humus. This is likewise true of the absorptive cells on 

 the rhizoids of mosses. 



Plants which lie flat against the bark of trees and have no connection with the 

 ground, so that they are unable to derive nutriment from it, have a veiy peculiar 

 method of maintaining themselves. Their roots, rhizoids, or hyphre, as the case may 

 be, either grow straight into the bark or are merely adnate to its surface. In 

 the latter case they are exposed on one side to the open air, and form more or 

 less projecting lines and ridges ramifying in all directions, often constituting a 

 regular trellis-work cemented to the bark. Sometimes, too, they are I'epresented 



