1938 Volume 8 Issue 2 Pages 124-144
1. In the present work were studied the penetration phenomena in Pseudoperonospora cubensis BERK. et CURT. inoculated on its hosts and other plants.
2. A large number of zoospores are often observed to encyst on the surface of their suspension drop on a slide or cover glass and the cysts produce the germ tubes uprightly above the surface.
3. The zoospores in a suspension drop on the lower surface of a cucumber leaf often swim along the boundary line of the epidermal cells, dashing themselves repeatedly against the stomata or the boundary line. This mode of the swimming movement appears to show the response of the zoospores to a stimulating substance diffused from the epidermis.
4. On the leaf of the cucumber zoospores encyst on the stomata and along the boundary line of the epidermal cells, being more numerously in the former than in the latter. Usually only a single cyst is found on a stoma irrespective of its size.
5. The zoospore encysted on the stoma sends out a penetrating hypha into the stomatal aperture and forms the vesicle in the substomatal cavity. Then the infection hypha developed from the vesicle reaches the parenchymatous cell and inserts the haustorium into it.
6. The cyst lying on the boundary line produces a slender hypha which is in a rare case successful in penetration through the stomata.
7. Penetration through the stomata is observed on the cotyledon. but not on the petiole, stem, and hypocotyle.
8. Inoculations on 73 species of Phanerogams ranging over 32 families including the Cucurbitaceae and on a fern plant show that 49 dicotyledonous plants (22 families) are penetrated by this fungus, always through the stomata and as easily on several plants as on the host.
9. On the upper surface of the epidermis stripped from the cucumber leaf encystment of zoospore and penetration take place in the same manner as on the intact one. On the other hand, when inoculated on the under surface of the stripped epidermis encystment and penetration are usually found on the boundary line.
10. The zoospores are attracted by the mesophyll cells of the cucumber leaf and after encysting upon their surface are able to produce the hyphae and haustoria.
11. On the lower surface of the cucumber leaf killed with boiling water the occurrence of encystment and penetration are less frequent than on the living one. The penetrations are obtained through the boundary line as well as the stomata, but the hyphae soon degenerate.