Cytology of Pythium ultimum , n. sp. 285 
first Indeed It was thought that they would prove permanently 
sterile. However, a culture six weeks old acquired patches of 
golden yellow colour, and an investigation of the reason for 
the change led to the discovery of both kinds of reproductive 
organs— conidia and ripe oospores, chiefly the former. Many 
of the cultures perish through drought before producing 
spores. We have here no doubt an Interesting example of 
the effect of abundant food material, as already noted for 
example by Klebs (’99) for Sctprolegnia mixta and by Goebel 
(’00) In the case of the protonema of Bryum pseudotriquetrum . 
The last culture set up was the thirty-first of a series, each of 
which was obtained by inoculation with a sterile fragment of 
its predecessor. The mycelium in this last culture is a 
direct descendant (by vegetative propagation) of that first 
obtained in August nearly five months ago, and still shows 
its tendency to remain sterile, while the abundant supply of 
nutriment remains unexhausted. 
These Interesting differences in the behaviour of the plant 
under different external conditions require further investiga- 
tion. I hope to be able, at some future date, to give a more 
satisfactory account of them in a paper on the physiology of 
the species. 
Dissemination of the spores. The conidia and oospores 
(or oogonia) are set free by the rotting away of the old 
exhausted mycelium. In pure cultures they may remain 
attached apparently for an indefinite period. In nature no 
doubt the spores are liberated and disseminated by the 
disintegration of the rotten substratum upon and in which 
they are found. No experiments have yet been made on 
their powers of resisting drought. The spores, however, do 
not appear to be adapted for dissemination by the wind. 
Potatoes have been purposely left exposed in the laboratory 
in places where enormous numbers of spores had been pro- 
duced, yet no spontaneous infection took place. The mode 
of dissemination indicated is, however, effective enough for 
a plant which vegetates in the surface soil on rotting vegetable 
and animal matter. 
