$4 MICROSCOPIC PLANTS. 21 
they are’capable of at once corroding, boring, and 
entering the epidermis of the leaf, or even the stem 
- or tuber itself. These zoospores are best seen when 
within the vesicle 7, where they arise from a differen- 
tiation of the contents; but when once set free (Z), they 
are, from the extreme rapidity of their movements, very 
difficult to make out. In almost half an hour they 
cease to move, their lash-like tails (cilia) disappear, 
and having burst at one end, a transparent tube is 
protruded, which is a similar mycelium in every 
respect with that produced by the simple spore, and 
which grows, branches, and fruits in a precisely simi- 
lar manner,” * 
But besides these zoospores and simple spores, 
the Potato fungus produces a third kind called 
“oospores,’ which were unknown until Mr. Smith’s 
careful study of the fungus by night and day through 
its various changes and developments revealed its 
existence, Referring to our fig. 21 again, we notice 
several large, round cells marked z, and some smaller 
k. The larger is the oogonium, and the smaller the 
antheridium. The oogonium is analogous to the 
ovary, and the antheridium to the anther, in flowering 
plants. These two bodies come in contact, and the 
antheridium pushes out a small tube which enters the 
cell-wall of the oogonium, and through it a portion 
of the antheridium contents is emptied into the oogo- 
nium. This fertilises the oospores which are contained 
within, and when the latter are mature the mycelium 
vanishes, and the resting spores are set free. It sinks 
into the earth, and remains quiet during the winter, 
* Worthington G. Smith, ‘‘Monthly Microscopical Journal,” 1875, 
