Abstract
Investigations on the nature of the thallus in the Peronosporales have been confined to Phytophthora and Peronospora, in both of which complicated conditions exist within a single species. Studies on the sexual relations of species of Pythium with single zo6spore cultures have not been reported. Long cultural experience with the genus Pythium, however, has given no indication that heterothallism exists in the group. In spite, therefore, of investigating the obvious homothallism of Pythium, preliminary studies on single zo6spore cultures were undertaken. The majority of species of this genus are monoclinous, that is, the antheridia arise in more or less close proximity to the o6gonium; in the diclinous species the antheridia arise from hyphae showing no traceable connection with the hypha bearing the oogonium or only extremely rarely. If heterothallic forms exist they would more likely be diclinous. The work of Ashby (1, 2, 3), Narasimhan (9), Leonianr (7) and others shows that in Phytophthora though some species are definitely homothallic others, such as the P. palmlivora Butler group, may possess definitely homothallic strains, homothallic strains with heterothallic tendencies, heterothallic strains, and neutral or apparently sterile strains. More recently, Helena de Bruyn (4) has reported similar complicated conditions in the thallism of Peronospora parasitica (Pers.) Tul. De Bruyn inclines to the view, held by Leonian and others, that the heterothallic strains are not strictly unisexual, but possess the potentialities of both sexes, one sex being ordinarily dominant. It is not intended here to discuss the views or evidence on the r6le chemical substances may play in heterothallism in Phytophthora (cf. 6, 8). The following table gives information on the species studied in the present investigation.
Published Version
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