i 
300 Wager. — On the Structure and 
estimate them at about one hundred. They are similar to 
the ordinary zoospores. Their movement in the water goes 
on for two to three hours. Then they slow down, the cilia 
disappear, the spore becomes immovable, takes a globular 
form and is surrounded by a cellulose membrane. Then 
a thin tube is put out, curved or straight, which attains a 
length of two to six times the diameter of the spore on the 
object-glass. The apex of this tube is swollen, and all the 
protoplasm of the spore gradually passes into it. There is 
never any further development of the tube on the object- 
glass. The tubes never penetrate roots. They enter only 
through the stomata on leaf or stem. When a drop of water 
containing zoospores is placed on the surface of the leaf of 
a cruciferous plant, the spores will be found after the lapse 
of some hours fixed on the stomata, their movement having 
ceased. A germ-tube is produced on the side of the spore 
nearest the pore, which it enters immediately, and plunges 
the swollen extremity into the air-cavity of the stoma. The 
spore never passes through the stoma. 
No further development of the tube takes place, however, 
and no mycelium is formed. It is only in the case of germ- 
tubes which enter the cotyledons that a mycelium is produced 
which spreads as the plant grows, and ultimately produces 
the characteristic white spots and pustules. 
Literature. 
The phenomenon of the formation of zoospores was 
described by Prevost as early as 1807. He describes them 
in a species found on the cabbage, which is evidently 
C. Candidas. His description is given very clearly and with 
much detail, and there is no doubt that he had under 
observation the formation of zoospores in this plant. 
L. R. and Ch. Tulasne in their memoir on the Ustilagineae 
and Uredineae (’ 4 7 ) refer to C. Candidas (Uredo Candida), 
and point out that the epidermis underneath which the 
