76 THE SAPKOLEGNIACE^ OF THE UNITED STATES, 



of the Saprolegnia of the salmon disease become motionless and encyst withont 

 swarming on leaving the sporangium. 



The zoospores of Achlya, Aphanomyces and Apodachlya only reach the water 

 just outside of the sporangium, and there become encysted, each one, as it leaves 

 the mouth, slipping into its place and rounding off at once, so that all the spores from 

 a sporangium form a hollow sphere or hemisphere, into which the tip of the sporangium 

 projects slightly (Figs. 10, 11). In other words, their cilia serve to carry the spores 

 only through the mouth of the sporangium. In Achlya the escaping spores foi-m a 

 column so compact that considerable space is left between it and the wall, and they 

 cling closely together during the entire emptying of the sporangium. It is always 

 noticeable that the spores in this column keep their long axes parallel with each 

 other. In AyJianomyces^ each of whose sporangia produces but a single file of 

 zoospores, the spores are compelled by the narrow space in which they are formed to 

 take a cylindrical shape. They pass in slow succession down to the mouth, and there 

 become encysted (Fig. 11). There is no crowding or clinging together, and the com- 

 plete emptying of the sporangium requires a much longer time than in the genera 

 already described. 



Hartog attributes this encystment of the spores at the mouth of the sporangium to 

 a mutual attraction between them which he terms adelphotaxy, and which is also 

 shown in their pressing closely together during their escape, in Achlya. It may well 

 be that in Aphanomyces they are prevented by their narrow quarters from showing 

 the same peculiarity inside of the spoi-angiura. If we accept the existence of such 

 an attraction, we must believe that it is stronger than the power of the ciUa to carry 

 the spores apart, or else that it is concomitant with a very transient development of 

 cilia. In the present state of our knowledge some assumption is necessary to account 

 for the phenomena which have been described. The spores are held together by no 

 material connections, and, at least in some species, are provided with cilia. That these 

 phenomena are dependent upon the life of the spore is shown by the fact that, if a 

 solution of osmic acid, which instantly kills and fixes the spores, be added at the 

 moment of escape, not only is their accumulation into a sphere stopped, but they are 

 freely separated and carried about by diffusion currents in the surrounding liquid. 

 It is hardly necessary to add that osmic acid would so harden any accompanying 

 mucus from the sporangium that the spores would be held firmly fixed if such sub- 

 stance were present, as some writers have believed. Hartog's assumption of a 

 mutual attraction between the spores seems as little open to objection as any that 

 can be suggested, and should be so stated as to include a tendency to place their 

 long axes parallel. This tcndenc}^ combined with the cfibrt to secure as much expo- 



