In order to test the relevance of previous results of this series to conditions operative in the intact plant, a technique was devised to simulate translocation of sucrose to mycorrhizal roots and to determine its morphological and biochemical destinations by the use of [14C]sugar. It was shown that the fungus can absorb sugar from the host and that its chemical destination is essentially similar to that of exogenous sucrose, namely a synthesis of mannitol, trehalose and glycogen. In connection with the possibility of reciprocal flow from fungus to host, the ability of uninfected roots to utilize the soluble sugars of the fungus was tested. Whereas mycorrhizas readily absorb and metabolize mannitol and trehalose, uninfected roots have almost no ability to utilize mannitol and are restricted in their utilization of trehalose, throwing considerable doubt on the possibility of sugar absorption by host from fungus. The hypothesis is developed that the fungus absorbs carbohydrates from the host and transforms them into reserve substances peculiar to itself, so maintaining a concentration gradient with respect to the host carbohydrate. The mycorrhizal system has the added advantage to the fungus that its reserve sugars are not readily re-available to the host. As polyols are abundant in many angiosperm parasites, the physiology of polyols in general is reviewed with the conclusion that the role postulated for this class of compound in mycorrhizas may be applicable to host-parasite relations in general.
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