LECTURE IX 

 STRUCTURE OF MYCORRHIZAE 



The Kinds of Mycorrhizae: — With insight which characterized 

 his work, Frank early stated that there are two principal sorts of 

 mycorrhizae, the coralloid sort found with forest trees, and the endo- 

 trophic which he illustrated from ericads. That general distinction 

 into basidio- and phycomycete types is found to hold generally good. 

 Yet it must be remembered that mycorrhizae are formed primarily 

 by the higher symbiont and that their form is determined by the vas- 

 cular plant producing it: the fungus is of secondary significance. One 

 fungus or another, or several fungi together, may invade the root, 

 but the mycorrhizal form will be essentially the same in all cases : its 

 form is characteristic for the higher symbiont rather than for the 

 fungus. This fact is emphasized by Woodroof (1933) whO' says: 

 ". . . it is seen that the influence of the fungus in the gross morphology 

 of mycorrhizal roots is slight. The presence of the fungus is to all 

 outward appearances merely incidental." 



Mycorrhizal Compared with Non-Mycorrhizal Roots: — 



Not all the rootlets of a mycorrhizal plant are necessarily mycorrhizal, 

 and a distinction must be made in the case of woody plants between 

 long-roots or "roots of extension" that grow rapidly through the soil, 

 and short-roots or small laterals which serve principally for intake of 

 materials from the soil. Woodroof (I.e.) calls attention to the fact 

 that not all short-roots are infected and that short-roots are not 

 started by the endophyte but, being formed, are invaded. The struc- 

 ture, whether invaded or not, is the same in both cases as regards gross 

 morphology. Long-roots are considered to be fungus-free, and when 

 short-roots are likewise uninfected they usually bear root-hairs ; but if 

 infected by symbiotic fungi, the short-roots become shortened and 

 swollen in their development. Yet Hatch (1937) presents some 

 evidence to indicate that mycorrhizal fungi stimulate their growth 

 and thereby increase the absorbing surface areas. The rootlet that 

 bears mycorrhizae is called a mother-root (following Noelle, 1910) : 

 the mother-root may renew its apical growth and extend out into 

 the soil as a pioneer-root. To illustrate, a simple mycorrhiza is a 

 mother-root bearing a very few elongate laterals ; a coralloid mycor- 



