THALLOPHYTES 



67 



of zoospores, a feature which distinguishes the genus from Peronospora, under 

 which the grape mildew was placed formerly. In the life history of Plasmo- 

 para, therefore, zoospores, characteristic of Oomycetes, have been eliminated. 

 This disease is of American origin and was unknown in Europe until American 

 grape stocks were introduced as a protection against the destructive phylloxera. 

 Peronospora. — These forms are very common parasites on ordinary vegetables, 

 as peiis, beans, spinach, etc., and differ from Plasmopara in the production of 

 laterally biciliate zoospores by the conidia, as in Albugo. 



(b) Zygomycetes 



General character. — This group of Phycomycetes is distinguished 

 from the Oomycetes in general by the estabhshment 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. i6r. — 



iagram 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 Mticor 

 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. 



