=395. Sexual reproduction of saprolegnia.=—When such cultures are older we often see large rounded bodies either at the end of a thread, or of a branch, which contain several smaller rounded bodies as shown in fig. 196. These are the oogonia (unless the plant is attacked by a parasite), and the round bodies inside are the egg-cells, if before fertilization, or the eggs, if after this process has taken place. Sometimes the slender antheridium can be seen coiled partly around the oogonium, and one end entering to come in contact with the egg-cell. But in some species the antheridium is not present, and that is the case with the species figured at 196. In this case the eggs mature without fertilization. This maturity of the egg without fertilization is called _parthenogenesis_, which occurs in other plants also, but is a rather rare phenomenon. [Illustration: 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.)] [Illustration: 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. [Illustration: 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.)] [Illustration: Fig. 205. Ripe oospore of Peronospora alsinearum.]