2JlS BOTANY OF THE LIVING PLANT 



brophytes (sec later) there appears to be complete dependence on 

 organic food derived from the fungus of the mycorrhiza. 



Two types of mycorrhiza are recognised. In the first the fungus 

 lives outside the root tissues of the plant with which it is associated ; 

 this is described as ectotrophic, and it regularly occurs in many trees, 

 e.g. Beech, Oak and Pine ; also in some herbaceous plants. In the 

 second type the fungus penetrates into the cortical cells of the root 

 of the higher plant, and this is styled endotrophic mycorrhiza. This 

 type is found in the Heaths and Orchids. But the two types of 

 mycorrhiza are linked by intermediate conditions, and in many 



imples of so-called ectotrophic mycorrhiza there is some degree of 

 penetration into the cells. 



Fig. 156. 



Part of the superficial tissue of a root of Sarcodes, covered by a felt of fungal 

 hyphae (h), in which the dark lines (r, c) are cast-off layers of the root-cap. The 

 outermost layer of cells of the cortex (c) is covered by a piliferous layer (e), but 

 the root-hairs are replaced by conical cells between which the fungal hyphae have 

 forced their way. (After Oliver.) Greatly magnified. 



(a) Ectotrophic Mycorrhiza. 

 Externally roots showing typical ectotrophic mycorrhiza appear 

 wrapped round by a covering of fungal origin and are short and 

 thick, and repeatedly branched. Sections show that the roots are 

 covered by a thick felt of matted fungal threads which usually envelop 

 the apical part of the root completely. The development of the 

 dense mantle of fungus may be due to the excretion from the root 

 surface of a substance favourable to fungal growth. Fig. 156 shows 

 part of a section through the root of Sarcodes, a native of 

 N. America, with ectotrophic mycorrhiza. The fungal mantle is seen 



