SYMBIOSIS 143 



group of vascular and in some cellular plants. In some 

 instances, as in many forest trees, the fungus lives in the 

 humus and forms a compact sheath round the root of its 

 host, which it supplies with a certain amount of water and 

 mineral food-constituents from the soil, and also organic 

 matter derived from humus. In such cases the fungus 

 does not obtain organic food from its host. In other 

 instances the fungal hyphae develop inside the root-cells 

 of the host, which supply them with organic food, and on 

 dying the hyphae furnish albuminoid substances that are 

 absorbed by the host. 



Such combinations of fungus hyphae and rootlets have 

 been termed mycorrhiza, are often fleshy and soft, and 

 assume a much-branched coraloid appearance, as in willows, 

 poplars, Monotropa hypopitys, etc. Others again cause 

 irregular swellings, as on the roots of alder, and are known 

 as mycodomatia or fungus-chambers, When the hyphae 

 are external as in the Abietineae, the mycorrhiza is said to 

 be ectotropic ; when developing in the cells, as in most 

 orchids, endotropic. Root-hairs are not formed on 

 rootlets that have been invested with hyphae. 



Mycorrhiza are most abundant when plants are growing 

 in situations where humus is abundant, and may be absent 

 from the same kind of plant in places devoid of humus. 



Stahl points out that the rate of transpiration determines 

 to a great extent the presence or absence of mycorrhiza in 

 a plant. Plants that transpire freely obtain their mineral 

 food independent of mycorrhiza, whereas those plants where 

 the transpiration current is slow require the assistance of 

 mycorrhiza to enable them to obtain the requisite supply 

 of mineral food and water. Hence we find growing in the 

 same wood some plants with, and some without, mycorrhiza. 



