394. Zoogonidia of saprolegnia.—The sporangium opens at the end, and the zoogonidia swirl out and swim around for a short time, when they come to rest. With a good magnifying power the two cilia on the end may be seen, or we may make them more distinct by treatment with Schultz’s solution, drawing some under the cover glass. The zoogonidium is oval and the cilia are at the pointed end. After they have been at rest for some time they often slip out of the thin wall, and swim again, this time with the two cilia on the side, and then the zoogonidium is this time more or less bean-shaped or reniform.
Fig. 199.
Fertilization in saprolegnia, tube of antheridium carrying in the nucleus of the sperm cell to the egg. In the right-hand figure a smaller sperm nucleus is about to fuse with the nucleus of the egg. (After Humphrey and Trow.)
Fig. 200.
Branching hypha of
Peronospora alsinearum.
Fig. 201.
Branched hypha of downy mildew
of grape showing peculiar branching
(Plasmopara viticola).
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.
