Fig. 202.
Gonidiophores and gonidia of potato blight
(Phytophthora infestans). b, an older stage
showing how the branch enlarges where it
grows beyond the older gonidium.
(After de Bary.)
Fig. 203.
Gonidia of potato blight forming
zoogonidia. (After de Bary.)
396. In [fig. 199] is shown the oogonium and an antheridium, and the antheridium is carrying in the male nucleus to the egg-cell. Spermatozoids are not developed here, but a nucleus in the antheridium reaches the egg-cell. It sinks in the protoplasm of the egg, comes in contact with the nucleus of the egg, and fuses with it. Thus fertilization is accomplished.
Downy Mildews.
397. The downy mildews make up a group of plants which are closely related to the water moulds, but they are parasitic on land plants, and some species produce very serious diseases. The mycelium grows between the cells of the leaves, stems, etc., of their hosts, and sends haustoria into the cells to take up nutriment. Gonidia are formed on threads which grow through the stomates to the outside and branch as shown in [figs. 198]-[201]. The gonidia are borne on the tips of the branches. The kind of branching bears some relation to the different genera. [Fig. 200] is from Peronospora alsinearum on leaves of cerastium; [figs. 197] and [199] are Plasmopara viticola, the grape mildew, while [figs. 198] and [202] are from Phytophthora infestans which causes a disease known as potato blight. The gonidia of peronospora germinate by a germ tube, those of plasmopara first form zoogonidia, while in phytophthora the gonidium may either germinate forming a thread, or each gonidium may first form several zoogonidia, as shown in [fig. 203].
Fig. 204.
Fertilization in Peronospora alsinearum; tube from antheridium carrying in the sperm nucleus in figure at the left, female nucleus near; fusion of the two nuclei shown in the two other figures. (After Berlese.)
