74 THE SAPKOLEGNIACE^ OP THE UNITED STATES, 



tion and the rounding of all their corners so that they come to lie nearly or quite free 

 from one another. In Achlya racemosa, and perhaps in some other species, the con- 

 traction is much less pronounced than is usual, and its spores do not commonly 

 appear distinctly separate in the sporangium. 



Up to this point our account may apply equally to all the species that have 

 been carefully studied. But we must now distinguish between those genera whose 

 spores normally escape from the apex of the sporangium and those of wdiich this is 

 not true. In those of the former class (A.cJ}lya, Saprolegnia, and allied genera), there 

 may often be seen some spontaneous movement among the spores, especially'' among 

 those near the tip. In case of the two genera just named, the apical papilla becomes 

 markedly more transparent and less sharply outlined, as to its terminal v/all. As the 

 spores acquire more exactly their ultimate form, the motion increases and the termi- 

 nal wall fades out until it is ruptured, and the spores rush through the opening (Fig. 

 8). Sometimes the wall seems to soften gradually until it yields to pressure from 

 within ; or again, the spore nearest the apex may be seen to enter the papilla and 

 apparently to force its way through the partly softened wall, thus making an opening 

 through which the rest rapidly follow. Other modes by which the opening is made 

 have been detailed by Rothert ('88), but need not be further discussed here. 



Some figures concerning the rapidity of the development of the zoospores have 

 been given by others, and I have made notes of the process in the four species chiefly 

 studied. Ward ('83) gives some observations on A. De Baryana Q'' jiolyandra "J 

 and A. ajnculata, and these accord with my own on A. Americana and racemosa as 

 closely as could be expected. There is considerable variation in the time occupied, 

 depending on the age and vigor of the culture, and doubtless on various undetermined 

 conditions. A general statement may, however, be based on the data at hand. The 

 first appearance of the clefts, v/hich mark the beginning of the formation of the 

 zoospores, usually occurs twenty or thirty minutes after formation of the basal wall. 

 From their appearance to the escape of the zoospores the time may be from twenty- 

 five minutes to an hour, but it rarely exceeds forty-five minutes. The emptying of 

 the sporangium is ordinarily accomplished in from forty-five to one hundred seconds. 



The zoospores are- ovatcor piriform, and their protoplasm is hyaline at the 

 smaller end, while in the rest of the spore it is very gi-anular, and contains two or 

 three contractile vacuoles (Fig. 8, z, and 9). The zoospores of the species of Sajyro- 

 legnia (Fig. 8,2;) and Pythiopsis (Fig. 63, z), and of some, probably of all, species 

 of AcJilya (Fig. 9) are provided with two cilia attached to the smaller, hyaline end 

 of each. The presence of these organs in the first-named genus has been recognized 

 since they were demonstrated by Thuret ('oO), but Cornu was the first ('72) to assert 



