Lecture IX —119— Structure 



The Exterior Surface: — Phycomycete and simple mycorrhizae 

 are usually smooth of surface and lack visible mycelial coating, while 

 coralloid mycorrhizae are often shaggy with hyphae. In the latter, 

 when the hyphae are densely interwoven they form a mantle that is 

 weft as closely as a tissue and when young may have a white satiny 

 surface, but when older becomes "fuzzy" with free hyphal ends. 

 Peyronel (1922) termed this mantle a micoclena, or it would perhaps 

 be better written "mycoclena", Greek for fungus-mantle; while 

 ZiEGENSPECK (1929) Called it a mycoderm, when it is a pseudotissue. 

 In both the smooth and the shaggy mycorrhizae there are doubtless 

 numerous hyphae that extend into the soil, passing from the soil cer- 

 tain materials into the interior of the root. Such hyphae have been ap- 

 propriately called Communication-hyphae. Being delicate, they are 

 inevitably broken in removing the mycorrhiza from the soil and could 

 be observed directly, if at all, only by some "glass-plate" method. 

 Communication-hyphae are the "root-hairs" of a mycorrhiza. 



But oftentimes the fungus produces short setose hyphae that 

 project evenly from surface of the mycorrhiza and simulate root- 

 hairs, — except that of course root-hairs are not developed from a 

 mycorrhiza, neither are they formed on a root-cap! Yet Gordon 

 (1936) describes and figures "root-hairs" over tip of a mycorrhizal 

 short-root. Presence of setae over apex of the rootlet or mycorrhiza 

 is indicative of their hyphal origin, and close examination with high- 

 power stereoscopic binocular microscope shows continuation of the 

 seta with a close-weft mantle hypha. Woodroof (1933) describes 

 pecan mycorrhizae with three sorts of setae on the surface, — flask- 

 shaped with long necks, with short necks, or stellate with intermixed 

 spines. Mangin (1910) also described and figured setae, as hairs 

 dilated at the base and tapered regularly to a point, — length 100-150 

 /x, diameter 5-6 jn at base. 



Or, hyphae may coalesce on or near the surface of the mycorrhiza 

 to form strands known as rhizomorphs which are similar to, but 

 usually smaller than, the rhizomorphs found in soil or under bark of 

 dead trees. 



It is quite possible that root-hairs and setose hyphae are present 

 simultaneously on a root, and fungal infection may be through root- 

 hairs; or it may occur directly through the epidermal wall. Root- 

 hairs are not developed, usually, to any extent on a plant provided with 

 mycorrhizae; yet their presence depends to a considerable extent 

 upon the soil in which the plant is growing, for in a forest the roots 

 will be mostly turned into mycorrhizae whereas in cultivated soil 

 root-hairs are more to be expected. Acid humus is not favourable to 

 growth of root-hairs, and when such are formed in so unfavourable 



