].] 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/, where they arise from a differen- 

 tiation of the contents; but when once set free (/i), 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 i t 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. 



