THALLOPHYTA: FUNGI 



117 



Summary. The Peronosporales are almost all internal parasites on 

 seed plants. They have a well-developed mycelium and small multi- 

 nucleate sporangia that, with few exceptions, are borne on erect spo- 

 rangiophores. The sporangia are almost always detachable and, after 

 dispersal by the wind, give rise to biciliate zoospores or, in some cases, 

 to a new mycelium directly. All members are heterogamous. The 

 oogonium produces only one egg, in the 

 formation of which the outer portion of 

 the oogonial protoplast is not included. 

 The male nucleus reaches the egg through 

 a fertilization tube developed by the 

 antheridium. 



6. Mucorales 



The Mucorales are the black molds, most 

 of which are terrestrial saprophytes living 

 on decaying vegetable and animal matter. 

 There are about 30 genera and 400 species, 

 common representatives of the group being 

 Rhizopus, Mucor, and Pilobolus. The 

 largest genus is Mucor, with about 50 

 species. 



Rhizopus. The common black mold that 

 grows on moist stale bread is Rhizopus 

 nigricans. It also occurs on fruits, vege- 

 tables, jelly, and other decaying organic 

 matter. The mycelium consists of a white 

 fluffy mass of profusely branched coenocytic 

 hyphae. These grow horizontally over the 

 substratum, sending into it tufts of short 



root-like haustoria through which food is absorbed (Fig. 92.4). Erect un- 

 branched sporangiophores arise in clusters from the mycelium at places 

 where the haustoria are formed. Each sporangiophore produces a large, 

 globular, terminal sporangium. In its development, the tip of the spo- 

 rangiophore enlarges as additional cytoplasm and nuclei pass into it (Fig. 

 93.4). Soon the peripheral part of the enlarging sporangium becomes 

 denser than the central portion and a line of vacuoles appears between 

 them (Fig. 93B). These two regions are then separated by a cleavage 

 furrow, arising from below, and finality by a dome-shaped wall. This 

 projects into the sporangium to form a columella (Fig. 92B). 



The portion of the sporangium lying between the columella and the 

 outer wall now undergoes a process of progressive cleavage, whereby it 

 becomes divided into numerous small, multinucleate protoplasts by 



Fig. 91. Plasmopara viticola on 

 the stem of grape. Sporangio- 

 phores bearing numerous spo- 

 rangia are emerging through a 

 stoma, X200. 



