1912] GRIGGS—RHODOCH YTRIUM 133 
did not. The process of conjugation is not different from that 
common in various algae. Two zoospores of approximately equal 
size approach (fig. 38) and lie alongside each other (figs. 39, 40); 
the plasma membranes separating them disappear, and within a 
few minutes the nuclei, which may be seen as clear central bodies, 
have fused into one (fig. 41). The two pairs of cilia remain distinct, 
and in the cases observed by me there persists a slight groove in 
the anterior portion of the zygote indicating the line of fusion. 
LAGERHEIM reports that both conjugated and unconjugated 
zoospores are able to infect the host. My own observations on 
this point gave no results. Repeated efforts were made to observe 
the process of infection, but for the most part the spores swam 
indifferently about the pieces of fresh ragweed which were placed 
on the slide with them. In some cases, indeed, the spores, both 
conjugated and single, settled down on such pieces of the host and 
became fastened to them with one or more cilia, but in no case did 
penetration occur. 
My attempts at reinfection on the living plant were similarly 
unsatisfactory. Out of numerous attempts, only three successful 
infections were secured. In these cases. the development of the 
young parasites was very rapid, but as the successful experiments 
were my first attempts in that direction, and as all efforts to repeat 
them failed, I do not feel warranted in reporting them in detail. 
One of the interesting questions which the failure of the infection 
experiments left unsolved is how the character of the young cysts 
is determined, that is, whether they are to develop into resting 
spores or into zoosporangia. Although this is connected with the 
seasonal cycle in North Carolina, there is no indication in LAGER- 
HEIM’S account that such is the case in Ecuador. By analogy with 
other forms, one might suspect that the zoosporangia spring from 
unconjugated zoospores and the resting spores from zoozygospores. 
But there is no definite alternation of generations, as in some such — 
forms. In any case, the character of the cyst appears to be deter- 
mined immediately on infection. As may be seen from the figures, 
the methods of penetration and growth are different from the very 
beginning, so that in the very youngest cysts there is no question 
whatever which are zoosporangia and which are resting spores. 
