Lecture IX — 117— Structure 



rhiza is a mother-root plus small coral-branched short-roots. But 

 in the case of annual plants or biennials, any of the secondary roots 

 may be infected, or even the adventitious roots ; while in several plants 

 with aerial roots, these are turned into mycorrhizae. 



External Form: — The form of a mycorrhiza is characteristic 

 for each species of plant and generally the form is constant for a 

 genus and even for a family. Thus, all cupulifers have a coralloid 

 sort of mycorrhiza ; Jitglans have simple, and Acer have necklace- 

 beaded mycorrhizae. Yet it must be noted that Melin (1925) states 

 that mycorrhizal form is greatly influenced by the sort of salts present 

 in the soil. The least complex form is the simple mycorrhiza which 

 consists of elongate monopodia! rootlets such as occur in Liriodendron, 

 Coriuis and Fraxinus. As described by Melin from pine heaths, 

 simple mycorrhizae may grow to 10 mm. length and 0.2 mm. diameter, 

 and are ordinarily without root-hairs. Apparent root-hairs on simple 

 mycorrhizae may on inspection turn out to be fungal setae, which often 

 simulate epidermal outgrowths. 



The coralloid mycorrhiza is said by Ulbrich (1924) to have been 

 first described by Hartig in 1851. It is branched freely like coral, 

 the "mother-root" bearing numerous short branches that, in compari- 

 son with the simple mycorrhiza, may grow to 1 mm. or more length 

 and 0.4 mm. diameter; i.e., they are short and thick. They are well 

 seen in pines, oaks, birches, and in the German are called "Gabel- 

 mykorrhizen", a shrub-like sort of structure. The racemose 

 mycorrhiza, as found in spruce and other forest trees, is formed by 

 lateral rootlets branching monopodially in two rows upon a main 

 axis. When coral-branches cluster thickly at one place to form a sort 

 of "witch's broom", the cluster is called a rhisothamnion; or, in the 

 German, a "Buschel". Rhizothamnia are seen on pine (Muller, 

 1902) and oak, and are said to be characteristic for Casuarinas 

 (MiEHE, 1918). Or, the cluster of short dense branches formed by 

 dichotomy may be weft about with mycelium to form a nodulous 

 lump called a tuberous mycorrhiza, or in the German, a "KnoUen- 

 mykorhiza". It is not truly a nodule, neither a tuber ; and is said to 

 have been first observed by Muller on Pinus montana. 



Pearl-necklace mycorrhizae are formed in yet a different way. 

 They commence as ordinary racemose mycorrhizae or perhaps as 

 widely spaced coral-branches but through intermittent growth succes- 

 sive additions are made and a constriction is left between each two 

 additions. Thus are developed the "pearl-necklace" beads so charac- 

 teristic of Acer, and found in various other plants. Such mycorrhizae 

 may be found on Pinus virginiana when the latter grows in droughty 



