180. The pin-mold fungi (Class Zygomycetes). Various fermentations or putrefactions affecting bread, preserves or other food, are often due to so-called "pin-molds" like the Mucor shown in Figs. 324, 325, 326. A spore falling upon the surface of some nutrient medium germinates by sending out one or more projections (Figs. 325, 3), and these finding abundant food available elongate and branch indefinitely so long as the conditions are favorable. Soon there may be as complex a system of branches as that in Fig. 324. A fungus thread is termed a hypha.1 The mass of hyphae forming the vegetative part of a fungal thallus constitutes a mycelium.2 The vegetative hyphae of Mucor form no partitions. hence we may consider the entire horizontal branch-work shown in Fig. 324 as one cell. Its position marks it as the pseudo-root of the plant, and for a while it is the only member developed. Pin-shaped vertical hyphae, which may be called pseudo-stems, arise into the air from the feeding mycelium, and the tip or "head" of each being separated by a convex partition, swells into a sporangium which becomes filled with a number of spores. These are soon scattered, and are then ready to produce new mycelia in the manner already described. Such non-sexual spores, serving as do swarm-spores for rapid multiplication, float like dust in the air, and may thus be distinguished as dust-spores.

1 Hy'pha - Gr. hyphe, a web.

1 My-ce'li-um - Gr. mykes, a fungus.

Fig. 324. Pin mold (Mucor Mucedo, Pin mold Family, Mucoracece). Plant showing the much branched horizontal mycelium from which arise pin like vertical hyphae (a, b, c, of different ages) that eventually develop dust spore cases at the tip. Somewhat magnified.

Fig. 324.-Pin-mold (Mucor Mucedo, Pin-mold Family, Mucoracece). Plant showing the much-branched horizontal mycelium from which arise pin-like vertical hyphae (a, b, c, of different ages) that eventually develop dust-spore-cases at the tip. Somewhat magnified. (Zopf.)

Fig. 325. Pin mold. 1, dust spore case, viewed as if cut vertically, showing the tip of the vertical hypha (c) projecting into the spore case; the spore case wall (m); and the numerous dust spores (sp), 225/1. 2, same from which the dust spores have been shed. 3, germinating dust spore, 300/1.

Fig. 325.-Pin-mold. 1, dust-spore-case, viewed as if cut vertically, showing the tip of the vertical hypha (c) projecting into the spore-case; the spore-case wall (m); and the numerous dust-spores (sp), 225/1. 2, same from which the dust-spores have been shed. 3, germinating dust-spore, 300/1. (Brefeld.)

Comparatively large and resistant zygospores are formed by the conjugation of special branches as shown in Fig. 326. The zygospore germinates by forming directly a pseudo-stem which bears a sporangium soon filled with dust-spores. The formation of zygospores at once suggests kinship with algae like Spirogyra, and it is believed that molds of the type here shown may have evolved from Chlorophyceae similar to the "pond-scums." The zygomycetes are fungi which produce zygospores.

Fig. 326. Pin mold. Formation and germination of zygospore. 1, two conjugating branches of the mycelium in contact. 2, separation of the tip of each by cross partitions, thus forming two conjugating cells (a, a) and two suspensors (b, b). 3, more advanced stage; warty thickenings have begun to form on the conjugating cells, which, however, are still separate. ripe zygospore (b) between the suspensors (a, a); the conjugating cells now having completely fused. 5, zygospore germinating by producing a vertical hypha with dust spore case at the tip. 1 4, magnified 225 diameters; 5, about 60 diameters.

Fig. 326.-Pin-mold. Formation and germination of zygospore. 1, two conjugating branches of the mycelium in contact. 2, separation of the tip of each by cross-partitions, thus forming two " conjugating-cells" (a, a) and two "suspensors" (b, b). 3, more advanced stage; warty thickenings have begun to form on the conjugating cells, which, however, are still separate. ripe zygospore (b) between the suspensors (a, a); the conjugating cells now having completely fused. 5, zygospore germinating by producing a vertical hypha with dust-spore case at the tip. 1-4, magnified 225 diameters; 5, about 60 diameters. (Brefeld.)