162 FUNDAMENTALS OF CYTOLOGY 



process the maximum cell size is restored. Diatoms with this type of 

 cycle are diplonts. The radially symmetrical Centrales have been 

 supposed to be haplonts, but recent investigations have shown that at 

 least some of them are diplonts. 



In the hlue-greeti algae no sexual reproductive cycle has ever been 

 discovered. The cell in this group multiplies by division and in some 

 genera, e.g., Oscillatoria, multicellular filaments are formed. These 

 filaments multiply by simple fragmentation. At the time of cell division 

 the problematical central body, which varies widely in distinctness and 

 resembles a nucleus to the extent of containing chromatin, may also 

 divide, but its exact nature and its function are not yet understood. 



Fungi. — The fungi, like the algae, show a rather bewildering array 

 of life-cycle types. This section must therefore be limited to a few of the 

 cytologicall}^ better known cases. These will illustrate the characteristic 

 reproductive behavior observed in the main subdivisions of this great 

 plant phylum. 



In the phycomycetes the m^xelium is coenocytic. In the order 

 Mucorales abundant asexual spores are produced in sporangia on the 

 mycelium. The sexual process involves the fusion of the multinucleate 

 contents of two gametangia. The fusion product becomes a thick-walled 

 resting zygospore. This germinates to form a mycelium or a "germ 

 sporangium," the spores from which produce new mycelia. The nuclear 

 behavior during this process is exceedingly difficult to follow, but recent 

 studies indicate several variations in the different species examined: 

 (1) the nuclei fuse in pairs and then undergo mciosis before the zygospore 

 matures (Mucor genevensis and others) ; (2) some of the nuclei fuse, the 

 others degenerate, and meiosis is delayed until zygospore germination 

 {e.g., Rhizopus nigricans); (3) the nuclei associate in groups in the zygo- 

 spore, and some of them fuse in pairs just before germination, meiosis 

 following in the developing germ sporangium {e.g., Phycomyces Blakes- 

 leeanus); (-1) there are no nuclear fusions at any stage {e.g., Sporodinia 

 grandis). In heterothallic species of Mucorales the plus and minus 

 tendencies are segregated in the spores from the germ sporangium. 



In the Saprolegniales there are motile asexual zoospores. There are 

 also well-differentiated eggs produced singl}^ or in groups in oogonia. 

 An antheridium applies itself to the surface of an oogonium and sends 

 in a tube which delivers a male nucleus to the single egg {Pythium). 

 When there are several eggs in the oogonium {Saprolegnia) , the antheridial 

 tube branches and delivers a nucleus to each of them. The zygote 

 takes the form of a resting oospore. When it germinates, meiosis occurs 

 (demonstrated in Achlya), indicating that these plants are haplonts. 



In the Blastocladiales there is a genus {AUomyces) in which several 

 strains have been shown to be diplohaplonts. The sexual plants bear 



