Endotrophic Mycorhiza of Ericacrez. 
2^7 
Metabolic- Exchangks between Host and Endophyte. 
Whether there is symbiosis or no, depends upon the exchanges 
of nutritive material between the two associated organisms. “When 
one plant is parasitic on another,” writes MacDougal (5), “ such 
conditions must be present as to cause water to flow from the host 
to the tissues of the parasite, carrying substances in solution.” 
In the case of Arbutus three parasitic relations may be 
considered:— 
(1) The fungus draws carbohydrates from the host. 
This is easily proved : a section of the stem is stained by Gram’s 
method, then treated, first with dilute potassium hydrate, secondly 
with weak hydrochloric acid (Mme. Gatin-Grusewska’s method). 
Starch grains are mostly dissolved, only the central part remaining. 
It is then possible to see the haustorial organs which the endophyte 
has sent toward the starch-grains to digest them (Fig. IV). 
Fig. IV. Endophyte in cells of leaf Arbutus Unedo . 
(2) The fungus root tubercles of Arbutus closely resemble 
those of leguminous plants, and the inoculated cells are near the 
surface of the host tissues. So it might be supposed that, here 
also, the fungus obtains free nitrogen from the atmosphere; but 
this, of course, can only be proved by cultivation of inoculated 
Arbutus in a soil free from nitrogen. 
