CYTOLiXiY OF REPRODUCTION IN PLANTS 



105 



((likaiya) migrate. As these hyphae grow, the paired iiiiclei (li\'i(ie 

 conjugatoly. Meanwhile the surrounding uninucleate hyi)lia(' continue 

 the development of the apothecium. 



Eventually an ascus is developed from the subterminal ctdl of each 

 ascogenous hypha. The two nuclei in this cell unite, and very soon 

 the resulting diploid nucleus undergoes three successive divisions, of 

 which the first two are meiotic in character. Spores are then formed 



Fig. 121. — Diagram of nuclear history in the life cycle of a heterothallic ascomycete. 

 Early and late stages of the cycle are shown, the early stages being in the lower portion of 

 the diagram and later stages in the upper portion. Long before the apothecium and the 

 spores are mature the sex organs have disappeared. The apothecium is composed of 

 uninucleate hyphae and binucleate ascogenous hyphae. The arrangement of successive 

 stages in the asci is arbitrary. In the ascus at the extreme left the last conjugate division 

 is being completed. Natural proportions are not represented. 



about the eight nuclei as centers. This involves a curious cytokinetic 

 activity of the astral rays remaining from the third nuclear division. 

 They curve around from each centrosome and cut out a portion of the 

 cytoplasm about each nucleus, leaving the residual cytoplasm to dis- 

 integrate. The eight ascospores sometimes lie in a row, and in some 

 species the positions of the mitotic figures in the three divisions that 

 produced spore nuclei have been so regular that it can be readily deter- 

 mined which portions of the chromatic matter now constituting the eight 

 nuclei were separated at each of the divisions. This has made it possible 

 in certain species to show by genetical evidence that disjunction of 



