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TEXTBOOK OP BOTANY 



in some cases the fungous threads penetrate the root cells. 

 It is thought by some who have studied these mycorhizas 

 that the fungi are not, strictly speaking, parasitic upon the 

 trees ; rather, there is a sort of part- 

 nership between a tree and the fungus 

 living in its roots. The fungus finds 

 a sheltered place in which to live, 

 and probably also uses as food some 

 of the substances in the root cells. 

 On the other hand, it is thought that 

 the fungous threads on the outside 

 of the root assist the root by absorb- 

 ing water and other substances, and 

 so take the place of root hairs ; it is 

 not unlikely too that the fungus digests 

 some of the organic substances, such 

 as dead parts of plants, that are in 

 the soil, and so changes them into a 

 form which the tree can use as food. 

 Thus, the tree seems to be indirectly 

 saprophytic; with the aid of the 

 fungus, it uses food materials which 

 without such help would be quite 

 useless to it. It is not only trees 

 that enter into this kind of partner- 

 ship with fungi ; some herbaceous 

 plants do the same thing. Among 

 the latter are many members of the 

 orchid family. 

 203. Roots of Epiphytes. — Epiphytes are plants that live 

 above the ground on the surfaces of other plants, but 

 are not parasitic upon them. Protococcus, when it lives 

 on the trunks of trees, is an epiphytic alga. A good 

 many mosses live on the trunks of trees. Epiphytic ferns 

 and seed plants are found mostly on the trunks and branches 



Fig. 114. — The Indian 

 pipe {Monolropa) ; a plant 

 without chlorophyl, which 

 lives upon organic sub- 

 stances in the soil with 

 the aid of fungi which 

 form a mycorhiza with 

 its roots. 



