LECTURE VI 



MYCOTROPHIC PLANTS AND THEIR 

 ENVIRONMENT 



Soil as a Mycotrophic Habitat: — There is perhaps no satis- 

 factory definition of soil. If soil is defined as the "unconsolidated 

 upper few feet of the earth's crust", then some mycorrhizae do not 

 occur in soil at all, for one may find them in pockets of humus formed 

 by decaying stubs on trunks of living trees. Such occurrences are not 

 uncommon with Acer ruhriim growing in swamps of the eastern 

 U.S.A. where the living tree will develop "necklace-beaded" mycor- 

 rhizae in pockets of humus on its own trunk from a root-branch de- 

 veloped by the trunk. Or, seedlings of other species, which have been 

 termed "pseudoepiphytes", may develop in such situations. Cordemoy 

 (1904) found that aerial roots of Vanilla form mycorrhizae in rotting 

 supports that are supposed to hold the plant up ; and the same thing 

 may be seen even better with pepper vines. Again, in a forest one may 

 find dozens of spruce or Tsuga seedlings growing on a partly decayed 

 log or stump ; and occasionally one finds a sapling that started in such 

 a situation and later extended its roots down into the soil, the stump 

 or log meanwhile rotting, leaving the sapling standing as it were on 

 stilt roots. All of these examples show that plants can grow for some 

 years in a flourishing condition without any contact with mineral 

 soil. If it is necessary for trees to have mycorrhizae in order to gain 

 inorganic salts from the soil, they must win the salts vicariously 

 for they have no direct contact with the soil, yet the seedlings flourish. 



In contrast, there are plants growing and producing mycorrhizae 

 in pure mineral soil which, as in Holler's (1902) sand, showed no 

 trace of humus. Or, mycorrhizal plants are found in agricultural soil 

 where humus and mineral portions are mixed together, although in 

 such situations the vesicular-arbuscular mycorrhiza is more likely to 

 be found. It may be said, then, that mycorrhizae are formed wherever 

 rootage organs grow in contact with appropriate fungi. 



Mycorrhizae and Soils: — Consequently, mycorrhizae are found 

 in a variety of soils. Von Tubeuf (1903) said: "Die Mycorhiza 

 findet sich auf Moorboden, im Waldhumus, auf nahrstoffreichen 

 Lehm — und gediingten Ackerboden und selbst im Bleisande des 



