KINDS OF MYCORRHIZAE 299 



Among Bryophyta internal mycelium has been found of com- 

 mon occurrence in certain genera, such as Buxbaumia and Tetra- 

 plodon, but whether this is a mutualistic relation is a point still 

 lacking experimental proof. 



In regard to mycotrophv among Pteridophyta there is also a 

 conflict of opinion. A Pythium-like fungus has been found in 

 the prothalli of several species of Lycopodium. Alycorrhizae 

 have been described as occurring in the root cortex of Ophio- 

 glossum and Botrychium. The endophytic mycelium from cer- 

 tain marattiaceous ferns is claimed to sporulate when isolated in 

 pure culture, an indication that the fungus may not be the true 

 symbiont. Rayner (1927) records the presence in Pferidium 

 aquilinium, a true fern, of typical endophytic mycorrhizae, with 

 arbuscules and sporangioles occurring within the root cortex. 

 The evidence that ferns are absolutely dependent upon the fungal 

 associate, in any case, may fairly be said to be not too convincing. 



Kinds of mycorrhizae. Various characteristics have been em- 

 ployed as bases of distinction in efforts to classify mycorrhizae. 

 Most commonly mycorrhizae are spoken of as either ectotrophic 

 or endotrophic. The ectotrophic group comprises those in which 

 the fungus remains in large part as a mantle over the exterior of 

 the roots, whereas the endotrophic group comprises those in 

 which the hyphae are within the host cells. In Frank's original 

 descriptions (1885) he directed attention to intercellular hyphae 

 beneath the mantle. These intercellular hyphae invest the cor- 

 tical cells and have been called the "Harti^-net." As mi^ht be 

 expected, forms intermediate between the true ectotrophic and 

 the true endotrophic exist. Such forms have been termed ectend- 

 otrophic. Hatch and Doak (1933), however, include the ec- 

 tendotrophic forms among the ectotrophic as transitional stages 

 between endotrophic and ectotrophic. 



Melin (1925) described three types of mycorrhizae on Scots 

 pine, the external form constituting the basis of separation. He 

 designated them as follows: (a) "Gabelmykorrhiza" (forked), 

 (b) "Knollenmykorrhiza" (knotted), and (c) "einfach Alykor- 

 rhiza" (simple). The first type is most common in nature, espe- 

 cially in woodland soils having an abundant layer of raw humus. 

 It is characterized by the possession of short, dichotomously 

 branched roots invested with mantles of various colors, the color 

 being determined by the species of fungus involved in its pro- 



