198 



BOTANY OF THE LIVING PLANT 



differences suggest different degrees of dependence for organic supply 

 upon some other source than Photo-Synthesis. That source is some 

 fungus which lives within the plant. The fungal filaments are able 

 to penetrate the cell-walls, and pass readily from cell to cell. Usually 

 they are massed in certain zones, especially in the cortex, and are 

 coiled within each cell, the protoplast and nucleus of which still retain 

 their vitality. This indicates that a mutual life is maintained a 

 condition of symbiosis. 



The Heath Family shows generally a mycorhizic condition. The 

 hyphae are present not only in the root of the Common Heather, but 

 also in the stem and leaves, and extend even to the ovary, where they 

 may be traced as infecting the seed-coat. The germ is free from them 

 in the seed, but infection takes place shortly after germination. 



FIG. 149. 



A single superficial cell of the young root of Common Heather (Calluna vulgtiris) 

 showing the endotrophic fungus, and its penetration of the cell-walls. (After 

 Rayner.) ( x 1500.) 



Without the fungus, as shown by pure sterile cultures, the seedlings 

 do not develop roots, and though they remain alive for months their 

 growth is stopped. On infection with the right fungus they develop 

 normally. Thus the synthesis of fungus and plant has been experi- 

 mentally accomplished. The conclusion is that the Symbiosis is a 

 necessary condition of normal life in the Common Heather. Similar 

 fungal infection of the seeds in the ovary is the rule in other Ericaceae 

 examined. 



The outside of the young root of Heather consists of a single layer 

 of large cells, with reduced cortex and a slender vascular strand within. 

 Root-hairs are absent. The outer surface is covered by septate and 

 branched hyphae, which often grow between the cells, while some 

 penetrate the walls. Within the cells they develop several stout 

 coils, with here and there short thick branchlets, completely filling 

 the cell (Fig. 149). In stem and leaf the fungus is found among the 

 superficial hairs, while hyphae penetrate even the cuticularised 



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