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, and potatoes, and 

 they cause "damping off" in a wide variety of 



SURFACE 

 HYPHA 



HAUSTORIUM 



Fig. 31-9. Some parasitic Phycomycetes, such as the 

 downy mildew shown here, possess specialized ab- 

 sorbing hyphae, called haustoria. These penetrate 

 into the cells of the host. (From The Plant World.) 



seedlings. In fact, it was one of the downy 

 mildews (Phytophthora infestans) that caused 

 the terrible Irish potato blight in 1845 and 

 led to the immigration of large numbers of 

 famine-ravished people into the United 

 States. 



On the basis of the general form and the 

 pattern of their reproductive processes, the 

 Phycomycetes seem quite closely related to 

 the green algae. However, the separation of 

 the two groups seems to have occurred in 

 very early times (Fig. 29-1 1). 



The Ascomyceies (Sac Fungi). This large 

 (35,000 species) class of the Eumycophyta in- 

 cludes many familiar and important forms. 

 Among the saprophytic species are the vari- 

 ous blue, green, and yellow molds (usually 

 either Aspergillus or Penicillium) often seen 

 on spoiling fruit; the red baker's mold, Neuro- 

 spora (p. 529); the various kinds of brewer's 

 and baker's yeasts (Fig. 10-2); and the rare 

 but delicious edible truffle. And among the 

 parasitic species are the powdery mildews, 

 which infect leaves, especially in lilacs, roses, 

 apples, and grapes; the ergot fungus which 

 infects rye and other cereals; the fungi caus- 

 ing chestnut blight and Dutch elm disease; 

 and one genus (Torula) of yeasts, which 

 may, on rare occasion, invade the skin and 

 nervous system in man, causing a serious 

 infection (blastomycosis). 



An important distinctive feature of the 

 Ascomycetes is a saclike reproductive struc- 

 ture called an ascus (Fig. 31-10). In the ascus. 

 four or (more frequently) eight haploid 



