MYCORRHIZA 



197 



cells of the roots but forms a mass of mycelial growth around the 

 root tips, entering the tissues largely between the cells; this type 

 of mycorrhiza is referred to as ectotrophic (Fig. 75). It is very 

 common among forest trees, especially evergreens. Other fungi 

 invade the cells within the roots, producing endotrophic mycorrhiza, 

 as in the case of the orchids (Fig. 76) and heather. Some fungi 

 produce slight penetration into the epidermis, and others invade 

 the cortical cells. Many of the fungi which develop upon roots 

 of woody trees belong to the Basidiomycetes, or fleshy fungi; 

 those that grow upon orchids are species of Rhizoctonia; and those 



Fig. 75. — Ectotrophic mycorrhiza developing on roots of Pinus sylvestris 



(from MeHn). 



on Ericaceous plants (heaths) are species of Phoma. In many 

 cases the specific fungi capable of bringing about mycorrhiza 

 formations are accompanied by other fungi in their invasion. 



The root fungi reach a high state of development in the case of 

 some of the orchids. These plants have very minute seeds con- 

 taining small amounts of reserve food material. The develop- 

 ment of the mycorrhiza appears to be indispensable to the growth 

 of orchids in nature. The fungi seem to increase the concentra- 

 tion of soluble organic substances about the embryo and also act 

 as root hairs for the young plants, furnishing organic and inorganic 

 nutrients during the early stages of growth. With the advance 

 in plant development, the fungi do not appear to be as essential. 

 Orchids have been cultivated from the seeds in the absence of the 



