Elementary botany . blight (Phytophthora in- Gonidia of potato festans). b, an older stage showing how the branch enlarges where blight forming zoogo- it grows beyond the older gonidium. (After de Bary.) nidia. (After de Bary.) the eggs mature without fertilization. This maturity of the egg withoutfertilization is called parthenogenesis, which occurs in other plants also, butis a rather rare phenomenon. 396. In fig. 199 is shown the oogonium and an antheridium, and theantheridium is carrying in the male nucleus to the egg cell. Spermatozoidsare not developed here, but a nucleus in the antherid
Elementary botany . blight (Phytophthora in- Gonidia of potato festans). b, an older stage showing how the branch enlarges where blight forming zoogo- it grows beyond the older gonidium. (After de Bary.) nidia. (After de Bary.) the eggs mature without fertilization. This maturity of the egg withoutfertilization is called parthenogenesis, which occurs in other plants also, butis a rather rare phenomenon. 396. In fig. 199 is shown the oogonium and an antheridium, and theantheridium is carrying in the male nucleus to the egg cell. Spermatozoidsare not developed here, but a nucleus in the antheridium reaches the eggcell. It sinks in the protoplasm of the egg, comes in contact with the nu-cleus of the egg, and fuses with it. Thus fertilization is accomplished. FUNGI: DOWNY MILDEWS. I85 Downy Mildews. 397. The downy mildews make up a group of plants which are closelyrelated to the water moulds, but they are parasitic on land plants, and somespecies produce very serious diseases. The mvcelium grows between the. Fig. in Peronospora alsinearum; tube from antheridium carrying in thesperm nucleus in figure at the left, female nucleus near; fusion of the two nucleishown in the two other figures. (After Berlese.) cells of the leaves, stems, etc., of their hosts, and sends haustoria into thecells to take up nutriment. Gonidia are formed on threads which growthrough the stomates to the out-side and branch as shown in The gonidia are borneon the tips of the branches. Thekind of branching bears some re-lation to the different 200 is from Peronosporaalsinearum on leaves of ceras-tium; figs. 197 and 199 are Plas-mopara viticola, the grape mil-dew, while figs. 198 and 202 arefrom Phytophthora infestanswhich causes a disease known aspotato blight. The gonidia ofperonospora germinate by a germtube, those of plasmopara firstform zoogonidia, while in phy-tophthora the gonidium may either germinate forming a thread, or eachgonidium may first form
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