26 



CHAP! I R 2 



life cycle comprise the beginning and end 

 of the male gametophyte stage. The haploid 

 microspore nucleus divides mitoticall) once. 

 to produce two haploid nuclei. One of these 

 nuclei elites not divide again and becomes the 

 pollen tube or vegetative nucleus. The 

 other nucleus divides mitotically once, so 

 that the gametophyte contains three hap- 

 loid nuclei (C); the two last formed func- 

 tion as sperm nuclei ( D. k ) . 



Near the base of the upper branches of 

 the corn plant are clusters of pistils, each 

 containing one diploid megaspore mother 

 cell, or megasporocyte. (The styles of the 

 pistils later become the silks. ) The mega- 

 sporocyte undergoes meiosis to produce four 

 haploid nuclei (Eh three of which degen- 

 erate (F). The remaining megaspore nu- 

 cleus divides mitotically (G), as do its 

 daughter and granddaughter nuclei (H). so 

 that eight haploid nuclei result (I). In the 

 embryo sac- (J), three of the eight nuclei 

 cluster at the apex and divide to form antip- 

 odal nuclei, two move to the center (polar 

 nuclei), and three (composed of two syn- 

 ergid nuclei and one egg nucleus) move to 

 the base of the embryo sac. The pollen tube 

 grows down the style to the embryo sac. 

 where one sperm nucleus fertilizes the egg 

 nucleus (K, L), producing a diploid (2N) 

 nucleus, and the other sperm nucleus fuses 

 with the two polar nuclei to produce a 

 triploid (3N) nucleus. With the occurrence 

 of this double fertilization the sporophyte 

 stage is initiated. Mitotic division of the 

 diploid nucleus (L) produces the embryo, 

 whiij the triploid nucleus develops into the 

 endosperm. The surface cells of the endo- 

 sperm comprise the aleurone, containing 

 aleurone grains and oil; the remaining endo- 

 sperm cells contain starch. The endosperm 

 is gradually digested to nourish the em- 

 bryo and seedling. The outer surface of 

 the kernel is the pericarp, diploid tissue 

 derived from the maternal sporophyte. In 



other words, a corn kernel has its pericarp 

 produced by one Sporophyte and its remain- 

 ing tissue h\ the sporophyte o\ the next gen- 

 eration. Development from embryo sac to 

 mature kernel requires about eight weeks 

 (during which the antipodal and synergid 

 nuclei or their deseendents degenerate); de- 

 velopment from the kernel to the mature 

 sporophyte occurs in about four months 

 (during which the first leaf completes the 

 digestion of the endosperm). 



3. Neurospora crassa "' 



Neurospora ("nerve spore") is a bread 

 mold whose haploid vegetative stage is com- 

 posed of threads or hyp hue which interweave 

 to form a mass, the mycelium. The hyphae 

 branch and fuse. Since the cell walls which 

 partition the hypha into cells are incomplete, 

 the cytoplasm of the filament is continuous. 

 Each hyphal cell is multinucleate. 



Cultures can be propagated asexually by 

 transplanting pieces of mycelium, or by 

 spores (conidia) which contain one or sev- 

 eral haploid nuclei. Sexual reproduction 

 (Figure 2-9) requires the participation of 

 molds of different mating type which pro- 

 duce fruiting bodies. A haploid nucleus 

 from one mating type enters and divides 

 mitotically a number of times in the fruit- 

 ing body of the opposite mating type. These 

 haploid nuclei pair with haploid nuclei of 

 the fruiting body and fuse to produce diploid 

 zygotic nuclei. Each zygotic nucleus then 

 undergoes two meiotic divisions to produce 

 four haploid nuclei; each of these divides 

 once mitotically to form a total of eight hap- 

 loid nuclei. The cytoplasm is then parti- 

 tioned to form eight ovoid haploid asco- 

 spores which are contained in a thin-walled 

 sac, the ascus. The mature fruiting body may 

 contain 300 asci, from which the ascospores 

 are released and carried in the air. Upon 



•' See Appendix on p. 29. tor references to experi- 

 mental methods, results, and literature. 



