. Principles of modern biology. Biology. 604 - Heredity and Evolution potatoes are both caused by parasitic slime molds. The evolutionary relationships of the slime molds represent a puzzling problem. It does seem clear, however, that the Myxo- mycophyta separated off from all other groups at a fairly early time (Fig. 29-11). The Eumyeophytes (True Fungi). This very large phylum consists of more than 75,(100 widely varying species. Most of these higher fungi are saprophytic, although there are quite a few parasitic species. A great major- ity are multicellular and in most cases the cellular or
. Principles of modern biology. Biology. 604 - Heredity and Evolution potatoes are both caused by parasitic slime molds. The evolutionary relationships of the slime molds represent a puzzling problem. It does seem clear, however, that the Myxo- mycophyta separated off from all other groups at a fairly early time (Fig. 29-11). The Eumyeophytes (True Fungi). This very large phylum consists of more than 75,(100 widely varying species. Most of these higher fungi are saprophytic, although there are quite a few parasitic species. A great major- ity are multicellular and in most cases the cellular organization is typical (that is, not syncytial). Usually the body, or mycelium, consists of a network of branched micro- scopic threads, or hyphae, which collectively may look like a Hulled out piece of absorb- ent cotton. The "fruiting organ," which usu- ally bears a large number of sporangia, fre- quently consists of a compactly organized system of hyphae of very definite shape and form. The gametes may be either motile or nonmotile; and sporulation may be either meiotic or mitotic. Four distinct classes have been evolved among the Eumycophyta. There are: 1. The Phycomycctes, or algalike fungi 2. The Ascomycetes, or sac fungi 3. The Basidiomycetes, or basidium fungi 4. The Deuteromycctes, or imperfect fungi The Phycomycetes. The algalike fungi are represented by about 500 species, which gen- erally are smaller and simpler than other eumyeophytes. Many Phycomycetes, like the bread mold (Fig. 12-10), are free-living sapro- phytes. However, there are quite a few para- sitic species, such as the downy mildews (Fig. 31-9) and the white rusts. Phycomycetes are generally more primitive than other eumyeo- phytes. Usually the hyphae arc syncytial, the mycelium is rather diffuse, and highly or- ganized fruiting organs are not developed. The downy mildews (Fig. 31-9) are dan- gerous disease-producing parasites. They cause considerable damage lo tobacco, grapes, lettuce, onions,
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