10 STRUCTURE AND CLASSIFICATION OF FUNGI 



called one of heterothallic conjugation. Heterothallism was first dis- 

 covered in fungi by Blakeslee. It is widespread among the fungi and 

 has been important in studying their life cycles, heredity, and varia- 

 tions since it makes it possible to carry, in many cases, the sexes in 

 separate pure cultures and to make crosses at will. Hybrids may 

 frequently be formed and certain fungi are being used by geneticists. 

 Their rapid growth and fructification and the fact that they can so 

 readily be grown in absolutely pure culture, under conditions con- 

 trolled at will, give fungi a decided advantage over seed plants, fruit 

 flies, etc. 



Heterogamy and Isogamy. In some cases the cells which fuse are 

 morphologically differentiated into male and female elements. In 

 some primitive forms, a passive egg cell may be fertilized by a motile 

 sperm cell. In higher forms the smaller or male cell is designated 

 an antheridium, the larger or female cell an oogonium. In many 

 fungi, however, the gametangia are not morphologically distinguish- 

 able. Being exactly alike in appearance, they cannot be called male 

 and female, but are designated + and — . Where the sex cells are 

 differentiated, reproduction is raid to be heterogamous ; when they 

 are alike the process is isogamous. Isogamy is often associated with 

 heterothallism, in which case the whole thallus is + or — . Instead 

 of the term sex, sign is sometimes used where there is no morpho- 

 logical differentiation into oogonia and antheridia. 



Multipolar Sexuality. Sex in the fungi is still further complicated 

 by the occurrence in some species of more than two "sexes." In 

 some of the Basidiomycetes one may find that of the four basidio- 

 spores, the nuclei of the mycelium from one may pair with the nuclei 

 in the mycelium from only one of the other three; thus mycelium 

 from basidiospore 1 may conjugate with 2 but not with 3 or 4, while 

 3 will conjugate with 4 but not with 1 or 2: Here there are obviously 

 not two but four "sexes." However, in some species only two types 

 of spores are produced from a single basidium. Mycelium from 

 spore 1 or 2 will mate with mycelium from spore 3 or 4, however 

 the hyphae may anastamose from any mycelium of the same species: 

 1 and 1, 1 and 2, 3, or 4, etc., but the nuclei will pair and divide con- 

 jugately only when the proper "sexes" come together. The mycelium 

 from a single spore in some species may fuse and divide conjugately 

 with mycelia from any one of the four spores from another individual 

 of the same species collected from some distant area. By matching 

 spores from many collections, a large number of races or strains 

 capable of fertile conjugation may be obtained. A somewhat similar 

 situation is found in some of the ciliated protozoa. Such a condition, 



