MYCORHIZA 77 



corolla bilabiate, white or tinged lilac, with purple veins, 

 central lobe of lower lip of corolla yellow. 



Bastard toad-flax (Thesium linophyllum, L.), a slender, 

 prostrate weed with narrow, one-nerved leaves and minute, 

 greenish flowers in terminal racemes. The rootstock is 

 woody and yellow, the fibres of the root are attached to the 

 roots of other plants growing in dry, chalky pastures, but 

 little or no injury is done to plants of economic value. 



MYCORHIZA 



It has long been known that the rootlets of various plants 

 are surrounded by a sheath consisting of fungus hyphae. 

 Frank considered this connection between the hyphae and 

 the root to be of a symbiotic nature, and termed the com- 

 bination of the two a mycorhiza. Such rootlets present a 

 thickened, coral-like appearance, and are often white, due to 

 the presence of masses of oxalate of lime adhering to the 

 hyphae. There are two principal types of mycorhiza, 

 ectotropic and endotropic. In the ectotropic form the 

 mycelium forms an external more or less compact sheath 

 round the tip of the rootlet, certain branches of the mycelium 

 penetrate between the cells of the outer layers of the cortex 

 of the rootlet, and push haustoria into the living cells of the 

 root. By this arrangement the host-plant is supposed to 

 be supplied to a certain extent with water, mineral food- 

 constituents, and organic matter, elaborated by the fungus 

 from humus, etc. Ectotropic mycorhiza occur in every 

 group of flowering plants, also in some cryptogams, and are 

 best developed in plants growing in situations where humus 

 is present ; in fact the same species of plant has mycorhiza 

 well developed when growing in soil containing humus, 

 whereas when growing in a sandy soil, devoid of humus, 

 mycorhiza are absent. Stahl has stated that the rate of 

 transpiration has a considerable influence on determining the 

 presence or absence of mycorhiza. Plants that transpire 

 freely have feebly developed, or no mycorhiza, being enabled 

 to obtain the required amount of mineral food due to their 

 rapid rate of transpiration, or the comparatively rapid rate 

 at which water containing the required salts in solution is 

 taken up by the roots. On the other hand, where the rate 

 of transpiration is normally slow, mycorhiza greatly assist 



