FOURTH LECTURE. 



SIGNIFICANCE OF MYCORRHIZAS. 



PROF. D. T. MACDOUGAL. 



By reason of the great adaptability of the seed plants, any 

 underground member, roots, stems, branches, or leaves may 

 come to serve as organs of absorption of liquid nutriment from 

 the soil in different species. Furthermore, because of this 

 readiness to adopt any method which will enable them to acquire 

 food more economically, the absorbing organs of an extremely 

 large number of species have acquired the habit of forming 

 symbiotic unions or partnerships with the hyphae of soil fungi. 

 Such unions are termed " mycorrhizas." But mycorrhizas are 

 not confined to the absorbing organs of the higher plants. The 

 roots of the sporophytic generation of a number of ferns and 

 fern-like plants also form similar structures, while the wide 

 occurrence of fungi in the thallus-like forms of the gameto- 

 phytes of the ferns, lycopods, hepatics, and equisetums, in such 

 manner as to constitute mycorrhizas, is well known. 



A mycorrhiza is classed as ectotropic or endotropic, according 

 to the manner in which the contact between the two plants is 

 made. Ectotropic mycorrhizas are those in which the fungus 

 forms a thick felt or sheath around the absorbing organs of the 

 higher plant. This form is always made by the union of a 

 fungus with true roots and never with stems, leaves, or pro- 

 thallia. The sheathing mass of hyphae may completely enclose 

 the root, or it may be absent from the apical region of this 

 member. In ectotropic forms the fungus does not actually 

 gain entrance to any of the living cells of the higher plant, 

 except in one or two families. The epidermal cells are papil- 



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