Physiology of the Saprolegniaceae . 379 
to reach maturity by inhibiting the production of profuse antheridial 
filaments. 
A further result that requires notice is the effect of rubidium and 
caesium hydroxides. These were added to the haemoglobin solutions, the 
same as the other salts. The results in all the cultures was a good vegeta- 
tive growth and a complete inhibition of sexual organs ; and not only these, 
but the sporanges, and even the gemmae, were almost entirely prevented 
forming. Even after two weeks in a 01 per cent, solution with haemoglobin 
only vegetative hyphae were present. Since rubidium was found by 
Benecke (’95) to be able to replace potassium as an essential element 
to growth, it was to be expected that it could replace it in the role of an 
aid to the formation of sexual organs. Of course it may be that potassium 
does not, as such, affect the production of oogonia to any great extent. 
The rubidium was not tried in the form of a nitrate. It is possible, too, 
that, like sodium, potassium merely performs the function of accelerating 
the process of reproduction ; but, if so, one might still expect the same 
property in rubidium. The view that the sexual organs are produced only 
after a good growth of mycelium would seem at first thought to find an 
exception in the case of rubidium, and in order to avoid that conclusion 
one must consider it as acting inhibitive in such cultures. 
III. 
The observation that certain species of the Saprolegniaceae never 
or seldom develop sexual organs has been made before. The subfamily 
Leptomitae, as used by Humphrey, includes only one species of Apodachlya 
which develops sexual organs, a new species described by Humphrey him- 
self. Maurizio (’96) found ‘ many species of Saprolegnia which yielded no 
oogonia even after three to six months of culture ’. I obtained from five 
different sources, two of which are not mentioned in the introduction, 
species from single zoospores which either formed no oogonia at all, or, in 
one case (I), only a few. One of these was presumably a Pithyopsis , 
although no final decision was reached as to its position. No oogonia were 
seen at all, either on this or on (L), in any of the cultures which were set 
up especially to determine this point. These species vegetated very 
luxuriantly, and just whether they were all distinct species or not is 
uncertain ; the same rate of growth and the sporanges are the only 
evidence to go by. We have here, however, a distinct physiological 
character by which these species of Saprolegnia are separable from those 
that produce oogonia so readily, like vS*. ferax , .S', mixta , and 5. hypogyna. 
The question here naturally arises whether these plants ever produce 
oogonia. I am inclined to think that they do, and that the conditions 
necessary are merely of a different kind than those so far tried on them. 
D d 
