SEXUAL REPRODUCTION 111 



leaves. Within 1 % hours the sporangia ^^ill mature, and they are 

 then dispersed. Several crops of sporangia are produced during 

 the night as the spent sporangiophores collapse and new ones 

 emerge in their places. This process will be repeated night after 

 night, except on dry nights. 



In P. tabacina, however, a single crop of sporangiophores and 

 sporangia is developed during the period between daybreak and 

 sunrise" The dependence of this organism upon temperature and 

 high relative humidity is indicated by the fact that epidemics are 

 suddenly checked with the advent of hot weather and that it has 

 been possible to induce fructification, under controlled condi- 

 tions, at any time throughout the year. Indeed, in North Caro- 

 lina sporangia were caused to be formed on 2 1 nights during the 

 month of August. 



Germination of the sporangia and infection are usually accom- 

 plished during the early morning. In all downy mildews, except 

 species of Peronospora, zoospores are normally produced. In 

 Peronospora, however, representing the most advanced terrestrial 

 habit, germ tubes are formed. In Peronoplasmopara, germination 

 of both types occurs, temperature being an important factor in 

 conditioning the type. Usually the zoospores are emitted singly 

 through a papillar opening, swim around for a brief period, 

 encyst, and then germinate by production of germ tubes. 



Sexual reproduction. The oogonia and antheridia form 

 within the tissues, arising on closely adjoining hyphal tips. De- 

 tails of their formation and of fertilization are contained in the 

 accounts of Wager (1900), Kruger (1910), Arens (1929, 1929a), 

 and McDonough (1937). Each organ is multinucleate after seg- 

 mentation from the supporting mycelium. Gradually the cyto- 

 plasm of the oogonium becomes differentiated into a central uni- 

 nucleate t^g and a peripheral periplasm containing the super- 

 numerary nuclei. All disintegrate except the t^^ nucleus. 

 Meanwhile, within the adjacent antheridium the nucleus of the 

 male gamete becomes differentiated, and the other nuclei, about 

 a dozen, disintegrate. A fertilization tube that pierces the 

 oogonial wall and periplasm is then produced. The male-gamete 

 nucleus passes through the fertilization tube into the ooplasm. 

 Nuclear fusion follows, whereupon a thick, resistant wall is 

 formed around the zygote. 



