Jan. 6, 1923 
A Phytophthora Footrot of Rhubarb 
13 
contents yellow. Next occurs a sudden bursting outward of the papilla, 
which can be seen in strip B. It was distinctly seen that there is no 
bursting of a membrane, but rather a stretching of the apparently gelati¬ 
nous substance of the papilla itself into a thin plasma membrane, which 
ordinarily remains intact, holding the contents together, until a large 
part or rarely all of the protoplasm is out of the sporangium. The mem¬ 
brane then gives way all at once, possibly by becoming dissolved rather 
than bursting at any one point, and the zoospores free themselves from 
one another and swim away in all directions. This stage is shown in 
Plate 11, C. By comparing the extremities of the strip, it will be seen 
that change is rapid at this stage. The pictures are taken at the rate of 
16 per second. At this stage also flagellae may often be seen lashing out 
on all sides of the protoplasmic mass at the mouth of the sporangium. 
With a large sporangium, which may contain as many as 64 zoospores, 
those that escape first in a single body usually become free and swim 
away before the last are out. This condition is evident in E. In such 
a case it is interesting to watch the amoeboid movements of the final 
parts of the protoplasmic mass. Figures F and G show the final stages 
of this very interesting phenomenon. Plate 8, B, also shows different 
stages of zoospore emergence, as drawn with the aid of the camera lucida. 
Zoospores are so active when first they become free that it is impossible 
to catch any but the faintest glimpse of the flagellae. Many observa¬ 
tions under different conditions, however, revealed bit by bit the various 
phases of their activity. Perhaps the best view of them in their normal 
activity was obtained in the slightly dull light of late afternoon, with the 
direct light from the blue sky and a 2.5-mm. Zeiss water immersion 
lens. This particular light appeared to provide the correct conditions 
for displaying the delicate flagellae. These observations were possibly 
still further favored by a low temperature, which appears to slow up the 
movements of the organism. 
The zoospores are the usual Phytophthora type, kidney-shaped, bicil- 
iate, contents granular, and nucleus indistinctly visible in the unstained 
specimens. They measure 8 to 9 by 11 to 13 ix while active, gradually 
rounding up as they lose their activity. Eventually they are globose 
and measure from 8 to 10 ju in diameter. The flagellae are attached 
close together in a longitudinal groove on one side. As the swarm spore 
swims along with its more pointed end forward, the anterior flagellum 
is the shorter, being approximately twice the length of the spore. 
It lashes violently back and forth. The posterior one is about 
2K times the length of the spore and drags along without much 
motion. As the zoospore gradually comes to rest, slight enlargements 
may be seen on the ends of the flagellae. As these droplets become larger 
the flagellum shortens by what appears to be a process of deliquescence. 
This has been observed in connection with some of the water molds an <5 
has evidently been observed in Phytophthora as well, for Sawada pic¬ 
tures it in connection with Phytophthora allii Sawada {22 , PL 1). The 
droplets approach the spore as it becomes quiet and round, and it is 
presumed that they drop off, though this has not been confirmed by the 
writer. One spore under observation for 15 minutes showed the drop¬ 
lets still not quite in contact with the spore, and it was lost before any 
further change took place. Most zoospores after having come to rest 
do not have these droplets attached. Plate 9, A to C, are camera-lucida 
drawings made from fixed materials. A slide showing swarm spores in 
various stages of activity was held for about 10 seconds over a bottle of 
