6 INTRODUCTION 



the groups of fungi except the artificial class called Fungi Imperfecti 

 which was estabhshed to include those fungi in which sexual reproduction 

 has not been discovered. The simplest type of sexual reproduction is the 

 union of two cells of equal size and to all appearances alike in all charac- 

 teristics. These two gametes may both be nonmotile (aplanogametes) or 

 motile (planogametes), as in the Yeasts and Chytridiales, respectively. 

 The zygote formed by their union may become a new one-celled plant 

 or may produce mycelium of various types. The gametes are more often 

 unequal in size and the motility may be limited only to the smaller one, 

 then designated as the male gamete or sperm. Still more often flagella 

 are lacking in both gametes and fertilization is brought about by the 

 union of the cells by dissolution of a portion of the intervening walls. 

 Sometimes the male nucleus is introduced into the female gamete, or egg, 

 through a tube extending into the latter. When the zygote is the product 

 of the union of clearly dissimilar gametes, forming a definite zygote cell 

 which often serves as a resting spore, it may be called an oospore. When 

 a similar resting spore results from the union of similar gametes it is 

 called a zygospore. It must be noted that the gradation between iso- 

 gametes (i.e., equal gametes) and anisogametes (i.e., gametes that are 

 dissimilar) is gradual. 



In some fungi, instead of producing definite gametes, any cell of one 

 mycelium may unite with any cell of another compatible mycelium, so 

 that no definite part of the fungus can be distinguished as a male or 

 female reproductive organ. This is especially true in the Ustilaginaceae. 

 However, many fungi do produce clearly distinguishable male and female 

 organs of reproduction. Very often the gametes may develop into new 

 plants without union, i.e., by parthenogenesis. This phenomenon is ob- 

 served from some of the simplest fungi up to many of the Higher Fungi. 

 Sometimes a vegetative cell may be substituted for the normal male 

 gametangium (antherid), the nucleus of this substituting cell functioning 

 in place of a normal sperm nucleus. So it comes about that, with partheno- 

 genesis occurring in many fungi and substitution of vegetative cells for 

 gametes in others, it is difficult to follow the evolutionary sequence of the 

 development of sexual reproduction in these organisms. 



The occurrence of the union of the sexual nuclei naturally leads to the 

 production of a diploid nucleus. In very few fungi does this nucleus 

 multiply in the diploid state (perhaps in a few Yeasts). Usually its first 

 divisions are meiotic, so that throughout the life history of the fungus 

 the nuclei are always haploid except immediately after the union of the 

 gamete nuclei. Yet among the fungi, particularly in the Ascomyceteae 

 and Basidiomyceteae, we find a contrast of mycelia that are diploid in 

 nature and haploid in nature although all the nuclei are haploid. These 

 are respectively the dicaryon and monocaryon phases of mycelia. Whether 



