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 Plzsmopara in the fact that the co- 

 nidia upon germination develop a mycelium directly. 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 differentiati <n 

 of individuals, although there is no distinct development of gam-.-tes. 



Mucorales. These are the black molds, which are mostly sa r - 

 phytes. The characteristic cobwebby, fleecy-white mycelium, c\-m 



FIG. 161. Mucnr: 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 

 Mucedo, 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. 



