Cell Division in Relation to Reproduction - 57 



CHLOROPLAST 

 NUCLEUS 



ZOOSPORE 



Fig. 3-14. Speculation in Ulothrix, a 

 filamentous green alga. 1, nonrepro- 

 duing cell of filament; 2-4, formation 

 and liberation of zoospores; 5-7, de- 

 velopment of a zoospore into a new 

 filament. 



suitable new medium, where it may begin to 

 grow and bud again. 



Many species of algae reproduce asexually 

 by means of free-swimming flagellated spores, 

 called swarm spores, or zoospores. At least 

 four, but frequently many, zoospores are 

 formed from a single mother cell, as in Ulo- 

 thrix (Fig. 3-14). In this plant the /oospores 

 free themselves and start migrating in all 

 directions. If one finds a favorable new loca- 

 tion, it settles down and develops into the 

 ordinary attached (sessile) form of the plant. 



Among the higher plants, including the 

 mosses, ferns, and seed plants, asexual repro- 

 duction by means of sporulation occurs at 

 regularly alternating periods during the life 

 cycle of each species. In these plants the num- 

 ber of spores produced by each of the spore 

 mother cells has become reduced to a stand- 

 ard of four. Each spore mother cell divides 

 only twice to produce its quota of spores. 

 These two divisions are meiotic in character, 

 and consequently the spores in all higher 

 plants are haploid cells. Each spore develops 

 without fertilization into a form of the plant 

 in which all the cells are haploid. Only later 

 in the cycle do such plants produce eggs and 

 sperm. Then fertilization occurs and the 

 diploid condition is restored (see Chap. 12). 



SEXUAL FORMS OF REPRODUCTION 



A great majority of living things, includ- 

 ing many unicellular and colonial forms, ex- 



hibit sexual reproduction. In these cases, 

 fertilization always occurs in one form or 

 another. 



The essential feature of fertilization is the 

 fusion of two haploid nuclei that usually 

 are recognizable as the egg nucleus and the 

 sperm nucleus. Ordinarily the resulting dip- 

 loid nucleus becomes the progenitor of all 

 the nuclei of the new individual. In more 

 primitive forms, however, it is not always 

 possible to distinguish between the egg and 

 sperm cells. Sometimes the two cells that 

 undergo fusion appear to be identical, and 

 therefore it is necessary to speak in broader 

 terms. The two haploid cells that come to- 

 gether in fertilization, whether similar or 

 dissimilar in appearance, are always desig- 

 nated as the gamete cells. The egg, when dis- 

 tinguishable, is called the female gamete, 

 and the sperm, the male gamete. Further- 

 more, the diploid nucleus created by the 

 fusion of the gamete nuclei is always called 

 the zygote nucleus; and the whole cell formed 

 at the time of fertilization is the zygote. In 

 evolution apparently sexual reproduction 

 came before any differences had developed 

 in the external appearance of individuals of 

 opposite sex, or between the gametes pro- 

 duced by the two cooperating parents. 



Isogamy vs. Heterogamy. A number of 

 examples can be found that illustrate the 

 foregoing principles. In Spirogyra and closely 

 related species, the ordinary cells of the fila- 

 ment are haploid cells, and any or all of these 



