130 THE SAPKOLEGNIACE^ OF THE UNITED STATES, 



in the present genus. Its oogonia are the largest of the genus and have rela- 

 tively smaller and sharper spines than the last species, while its antheridia are 

 well developed and always present. But its chief peculiarity lies in its parasi- 

 tism upon Algae, in which it is unique among Saprolegniacece and recalls the 

 related genus Pytliium. Its host-plants are Spirogyra and Zygnema, whose cells 

 it rapidly destroys. It has been observed as yet only in Europe. While struc- 

 turally distinct from the last species, and shown by DeBary to be also physio- 

 logically so, this must be regarded as a plant of the same type, which has 

 acquired the parasitic habit. 



Aphanomyces scabek DeBary ('GO). 



111. : DeBary, '60, PI. XX, Figs. 14^16. 

 DeBary, '81, PI. VI, Figs. 30-36. 

 PI. XX, Figs. lOS-111. 



Zoosporangia very long. Oogonia small, terminal on short branches, or on main 

 hyphse, globular; their walls with numerous short spines or prominences, or merely 

 irregularly roughened. Antheridia on branches of androgynous or diclinous origin, 

 small, not on all oogonia. Oospores single, globular, centric, 16 to 18//. in diameter. 



Massachusetts — Amherst. Europe. 



The spiny form of this species (Figs. 108, 109) appeared in a single culture from 

 dead leaves and slime taken from a ditch in Amherst, and the merely rough form 

 (Figs. 110, 111) was obtained later from a mass of Spirogyra which grew in the same 

 ditch, at a point a few rods away from the source of the first. 



Both forms agree closely except in the roughnesses of the oogonia, which 

 may be very slight or may take the form of short and rather sharp spines. But the 

 species is always known by the smaller oospores and by the reduced size and number 

 of the antheridia. These latter are whoUv wanting on half or even more of the 

 oogonia. In my few cultures they have been rather less abundant in the smoother 

 than in the spiny form, and the two extreme types of oogonia have not appeared 

 together. It may therefore prove justifiable to regard the smoother form as a distinct 

 variety, but further evidence is needed on this point. The character of the present spe- 

 cies points to the conclusion that it should be regarded as the least typical of the 

 genus, representing a degeneration in both sexual organs from the type of A. stellatus. 



