I] GENERAL CHARACTERS OF THE FUNGI 3 



of a female and a vegetative nucleus, is very common among fungi, and a 

 complete disappearance of even this reminiscence of a sexual process is by 

 no means rare. It has been suggested that the variety of food material which 

 fungi as parasites and saprophytes obtain from their substratum may make 

 the stimulus of fertilization less important, and it is possible also that among 

 these plants competition is less severe than among holophyta or holozoa. 

 At any rate the group shows a progressive disappearance of normal 

 sexuality. 



The sexual fusion or its equivalent is followed in all investigated cases 

 by a reducing division or meiotic phase, so that, as in other plants or 

 animals, the number of chromosomes is doubled in fertilization and sub- 

 sequently halved in meiosis, and diploid 1 and haploid phases follow one 

 another. 



The meiotic phase is usually associated with spore-formation which, in 

 many of the lower fungi, takes place on the germination of the zygote. In 

 a much greater number of cases a period of vegetative development inter- 

 venes between the association of the nuclei in fertilization or otherwise and 

 chromosome reduction, and we have a well-marked alternation of generations 

 in which a haploid gametophyte bears the sexual cells or their equivalent, 

 and a diploid sporophyte gives rise to spores which in turn constitute the first 

 stage of a new gametophytic generation. It is not at all uncommon to find 

 several sporophytes arisingfrom a single gametophyte, and the gametophytic 

 mycelium frequently sends out branches which grow around and protect the 

 sexual cells and their products. Where fertilization or any equivalent process 

 has wholly disappeared we may expect to find a similar morphological 

 alternation of generations, though without the corresponding cytological 

 changes; but in some cases, as in the large group of Fungi imperfecti, a 

 sporophyte is no longer developed, or at any rate has not been identified. 



Spores and Spore mother-cells. In the higher fungi the characteristic 

 spores of the sporophyte, with the development of which meiosis is definitely 

 associated, may be produced either endogenously as ascopores in a mother- 

 cell of definitely restricted size termed an ascus, or exogenously as basidio- 

 spores on the exterior of a cell or row of cells known as a basidium. The 

 asci or basidia are frequently arranged in parallel series forming a fertile 

 layer or hymenium sometimes of considerable extent. They arise from a 

 sub-hymenial layer immediately below the hymenium, and among them 

 are interpolated elongated vegetative cells or paraphyses, which are probably 

 concerned in their nutrition and perhaps assist spore dispersal by keeping 

 the mother-cells separate. The ascus and basidium and their products have 

 long been recognized as essential features in classification. 



1 The diploid unit may be defined as a protoplast the nuclear content of which includes the 

 double number of chromosomes. 



