THALLOPHYTES 67 
as in Albugo, a feature which distinguishes the genus from Peronospora, under 
which the grape mildew was placed formerly. This disease is of American origin 
and was unknown in Europe until American grape stocks were introduced as a pro- 
tection against the destructive phylloxera. 
Peronospora. These forms are very common parasites on ordinary vegetables, 
as peas, beans, spinach, etc., and differ from Plasmopara in the fact that the co- 
nidia upon germination develop a mycelium dfrectly. In the life history of Pero- 
nospora, therefore, zoospores, characteristic of the Oomycetes, have been eliminated. 
(b) Zygomycetes 
General character. This group of Phycomycetes is distinguished 
from the Oomycetes in general by the establishment of the aerial habit; 
by the elimination of zoospores; by so-called isogamy, so far as the 
sexual reproduction is known; and perhaps by the sexual differentiation 
of individuals, although there is no distinct development of gametes. 
Mucorales. These are the black molds, which are mostly sapro- 
phytes. The characteristic cobwebby, fleecy-white mycelium, com- 
FIG. 161. Mucor: diagram showing mycelium and sporophores. 
posed of large, often glistening, profusely branching hyphae, is very 
common on decaying material, stale bread (kept moist and warm), 
fruits and fruit juices, etc. The ordinary form on dung is Mucor 
MucedOj while the common bread mold is Rhizopus nigricans (fig. 161). 
Sporangia. The stout sporophores bear globular sporangia, the 
spores and stalk being dark or even black, suggesting the name black 
mold. After the terminal sporangium cell is cut off, the separating wall 
bulges into the sporangium cavity, forming the so-called columella 
(fig. 162). The sporangium wall finally becomes mucilaginous and the 
spores are set free and dispersed, forming new mycelia directly. 
