292 BOTANICAL GAZETTE [October 



in various stages of emergence. This process, as shown in figs. 

 10-13, closely resembles the previous emergence of the zoospore 

 from the retained sporangiospores. The zoospore emerging from 

 the cystospore moreover is exactly like the original zoospore 

 which emerged from the sporangiospore. The cultures in which 

 this additional emergence of the zoospore was observed were 

 derived from single spores, and their purity was beyond question. 



To appreciate the significance of this repeated zoospore emer- 

 gence in Dictyuchus, it is necessary to consider briefly the 

 corresponding phenomena in certain related types. The genera 

 of the Saprolegniaceae, as is well known, are distinguished by the 

 characteristic peculiarities of their non-sexual reproduction, since 

 a regular and distinct cycle of non-sexual spore production marks 

 each separate genus. It has been customary to arrange the genera 

 in a series in accordance with the degree of simplicity or the com- 

 plexity of their cycles. Disregarding the question whether such 

 a series represents the elaboration of a simple type or the simpli- 

 fication of a complex one, we may first consider Saprolegnia, 

 which has the most extensive cycle. Since in the other main 

 genera the cycles are less extensive in increasing degrees, we may, 

 in accordance with this, arrange a convenient series: Saprolegnia, 

 Achlya, Thraustotheca, Dictyuchus, and Aplanes (text fig. i). 



The characteristic cycles of non-sexual reproduction in these 

 5 genera are shown in the accompanying diagrams, which follow 

 the accepted descriptions for all but Thraustotheca. In this genus 

 the author has found that the non-motile sporangiospores swell 

 and escape by bursting the enveloping sporangium wall, after 

 which they closely resemble the escaped sporangiospores of Achlya 

 in their further development. The genera form a series repre- 

 senting a gradual decrease in the extent of the cycle of non-sexual 

 spore formation. The series ranges from Saprolegnia, with its 

 swarming of primary and secondary zoospores, through successive 

 stages to Aplanes, which is believed to lack both of these phases 

 of zoospore activity. 



In the repeated emergence of its zoospores this species of Dictyu- 

 chus shows a distinct departure from the cycle of spore formation 



