130 BOTANICAL GAZETTE [FEBRUARY 
whether parasitized or not. But before these leaves wither, the 
parasites they contain ripen and discharge their zoospores, which 
carry the infection to the younger parts nearer the growing point 
of the host. In this manner infection is carried to successively 
higher and higher levels of the growing plant, until the host is 
often red with parasites. 
LAGERHEIM believed that the cysts arose not only by direct 
infection but also by proliferation from the mycelium of old ones. 
If this occurred, a single infection might, by repeated proliferation, 
infect every part of the host plant. But in the form on Ambrosia 
no indication of such proliferation was found. Nowhere among 
the rhizoids were any indications observed of the formation of new 
growing points or other signs of proliferation. During the whole 
of the growth period the parasite is strictly unicellular, with a single 
nucleus in the body of the cyst, and when nuclear division begins 
preparatory to sporulation, the nuclei do not wander into the 
rhizoids. In sectioned material only a small proportion of the 
parasites are so oriented that a single section passes centrally 
through the whole cyst. But in no case, where the series was 
complete, was there any difficulty in finding the external opening 
of any zoosporangium, whereas if proliferation had been occurring, 
numerous partially formed cysts which had not yet grown out to 
the epidermis should have been encountered. In those parasites 
which become resting spores the independence of the cysts cannot 
be demonstrated by finding their external openings, because, on 
account of the narrowness of their necks, only a small proportion 
of them can be followed to the exterior. If proliferation occurred, 
the new cysts could become nucleated only by migration of nuclei 
through the rhizoids. But not only do the nuclei of the resting 
spores remain undivided, but they have not been seen to wander 
from their central position in the middle of the cyst, and they are 
so large that it is difficult to imagine them squeezing through the 
rhizoids. 
_ Although the parasites are so abundant as almost to cover the 
host plant, and the rhizoids destroy the cells which they penetrate, 
the vigor of the plant is little impaired. But when infected rag- 
weeds are transplanted, it is difficult to prevent the parasitized 
