MYCORRHIZA 251 



Orchids) secrete special enzymes, which are able to convert some of 

 the insoluble organic constituents of the humus, such as the starch 

 contained in the dead vegetable tissues, into readily diffusible 

 compounds. 



The members of a different class of green humus-plants comprising 

 the Cupuliferae and Betulaceae besides many Conifers instead of 

 producing numerous root-hairs, provide themselves with a large absorb- 

 ing surface by entering into symbiotic association with a Fungus- 

 mycelium. At the beginning of the present chapter, it was pointed out 

 that the filamentous mycelium which is the normal form of plant-body 

 among Fungi primarily represents an organ of absorption. The aforesaid 

 woody plants, and also many herbaceous saprophytes, make use of this 

 highly efficient foreign absorbing structure for their own purposes. In 

 such plants, the absorbing roots are completely enveloped from the 

 tip downwards in a perfectly continuous mycelial sheath composed 

 of densely interwoven hyphae ; individual branches of this sheathing 

 mycelium penetrate between the hairless absorbing cells of the root, 

 which thus become likewise more or less enveloped in hyphae. The 

 sheath, on the other hand, usually sends forth a number of hyphae 

 from its outer surface, which pervade the surrounding humus in all 

 directions. The symbiotic hyphae are more efficient than ordinary 

 root-hairs, not only quantitatively, on account of the larger surface 

 which they expose, but also qualitatively, because they have a greater 

 inherent capacity for utilising the various constituents of the humus. 

 Fungus-hyphae are, in other words, more thoroughly equipped than 

 root-hairs for a saprophytic mode of life. 



The water and the soluble inorganic and organic food constituents 

 absorbed by the fungus-hyphae are taken over from them by the outer- 

 most cell-layer of the mycorrhiza 1 20a as this particular type of root is 

 called whence they pass to the ordinary conducting tissues of the 

 root ; this outermost layer, of course, corresponds to the piliferous layer 

 (rhizodermis) of an ordinary root. Frank was able to show experi- 

 mentally that the association with a Fungus-mycelium is actually 

 beneficial to a mycorrhizal plant. Young plants of Beech were grown 

 in pots containing forest-soil rich in humus. A certain number of the 

 pots were previously sterilised by heating to 100 C. The plants 

 growing in the unsterilised humus soon developed the characteristic 

 mycorrhiza, and continued to thrive; those which were planted in the 

 sterilised soil, on the contrary, formed no mycorrhiza and gradually 

 sickened and died. 130 



Many Vascular Cryptogams (prothallia of Lycopodium, Hymeno- 

 phyllaceae) and Mosses likewise lead a more or less saprophytic 

 existence. The author has shown that the rhizoids of several Mosses 



