116 PLANT MORPHOLOGY 



The oogonium is a globular multinucleate cell, cut off by a cross wall from 

 the swollen end of a hypha (Fig. 905). The cytoplasm becomes differ- 

 entiated into a peripheral zone, the periplasm, and a central denser 

 region, the ooplasm, which becomes the egg. At first both regions are 

 multinucleate, but later all nuclei degenerate except a single nucleus in 

 the ooplasm. The antheridium, appearing on a separate hypha, is a 

 slender multinucleate cell. After coming in contact with the oogonium, 

 it sends into it a fertilizing tube that extends into the egg, where a male 

 nucleus and a small amount of cytoplasm are discharged. Following 

 fusion of the male and female nuclei, the periplasm is used up in the 

 formation of a heavy wall around the zygote. The zygote is finally freed 

 by decay of the host tissues and, after undergoing a period of rest, gives 

 rise to more than one hundred biciliate zoospores, each of which may, 

 under appropriate conditions, produce a new mycelium. 



Albugo bliti, a species common on the pigweed {Amaranthus) , differs 

 from Albugo Candida in several respects. Periplasm and ooplasm are 

 differentiated but the latter remains multinucleate. The entire contents 

 of the antheridium are discharged into the egg and each male nucleus 

 fuses with a female nucleus. In Albugo portulacae, which lives on the 

 common purslane (Portulaca), multinucleate pairing and fusing also occur 



(Fig. 90C). 



Other Downy Mildews. The Peronosporales include genera that 

 bear sex organs like those of Albugo, but differ in the way their sporangia 

 and spores are formed. Some of these are of considerable economic 

 importance. A species of Pythium is frequently the cause of a disease of 

 seedlings known as damping-off. It is particularly common in green- 

 houses and other warm, moist places. Pythium is intermediate between 

 the Saprolegniales and Peronosporales in that it produces zoospores in 

 both permanent and detachable sporangia. 



Phyfophthora infestans causes a serious potato disease called late blight, 

 while another species, Phytophthora cifrophtJiora, is responsible for the 

 brown rot of lemon. Plasmopara viticola causes downy mildew of the 

 grape, a very destructive disease. In both Phytophthora and Plasmopara 

 the internal mycelium sends erect sporangiophores to the surface of the 

 host (Fig. 91). Instead of forming blisters, as in Albugo, the sporangio- 

 phores push out through the stomata and bear solitary terminal sporangia 

 on branches. The sporangia, which are shed without opening, are carried 

 by the wind to uninfected hosts, where each produces several biciliate 

 zoospores. These form a new mycelium within the leaf. 



Peronospora is a large genus of about 60 species, some of which are 

 parasitic on various garden vegetables, such as cabbage, spinach, onion, 

 pea, etc. It is of interest in that, in many species, no zoospores are pro- 

 duced, the detachable sporangia giving rise to new mycelia directly. 



