MINUTE FUNGUS ON A FREE-SWIMMING ALGA. 121
The most interesting thing about these specimens was,
however, that they were mostly parasitised by a minute fungus
belonging to the group of the Chytridiales, and I was fortunate
enough to be able to watch the process by which this was brought
about.
To those who only think of fungi in connection with the large
terrestrial forms, and even to those who include in the picture
the smaller forms like rusts and moulds, it may be surprising
that there should be fungi which are wholly aquatic and which are,
moreover, so small that they can live upon organisms no bigger
than the Chlorogonium just referred to. Most of the members
of the Chytridiales are, however, of this character. It is true
that some are not aquatic, but most of them are, and they are
usually found as parasites on various kinds of green alga;,
especially the filamentous sorts. They have also been found on
floating pollen grains, on small aquatic animals and on other
aquatic fungi such as Saprolegnia. They are very simple in
character as a rule, having only a rudimentary mycelium, if any,
and are sometimes reduced to nothing but a sporangium.
My attention was called to the presence of the Chytrids by
noticing that many specimens of Chlorogonium had attached to
them little colourless spherical bodies usually containing a single
highly refractile globule. In one case, which luckily came to
rest, I was able to see that the largest of these little spheres,
about a three-thousandth of an inch in diameter, had granular
contents and a slight protuberance of the cell wall at one point.
It was fairly evident, therefore, that it was a developing zoo-
sporangium. It was first seen in the morning: by the evening
the contents showed some two dozen fairly regularly spaced
refractile globules, each of which, no doubt, indicated a zoospore
in the course of formation. The next morning it was found that
the protuberance had opened like a mouth and that all except
one of the zoospores had escaped. The zoospores outside the
case of the zoosporangium were actively moving about, but had
not travelled very far away. The zoospore that remained inside
was also actively moving about, sometimes even getting its
flagellum out of the mouth of the case. By three p.m. it also had
managed to get out.
As regards these zoospores a word or two may be said here.
They were very small colourless bodies only about two microns