472 



BIOLOGY AND HUMAN LIFE 



346. Fertilization. In some of the seaweeds and other classes 

 of plants, gametes are produced in special organs or on special 

 parts of the plant body. This shows a division of labor be- 

 tween vegetative and reproductive regions or structures. There 

 is a further tendency for the two gametes that take part in con- 

 jugation to become different from each other. In the bladder 

 wrack, or rockweed, which is common on our coasts, some of the 

 bladders on the tips of the fronds bear organs that discharge 



a c d 



Fig. 194. Reproduction in rockweed, or bladder wrack 



a, expansions of the rockweed containing the gamete organs; b, section of an egg- 

 bearing organ; c, the large gamete, or egg, with large, distinct nucleus and food gran- 

 ules; d, the small gamete, or sperm, having the shape of a pear and bearing motile 

 cilia. Sperms swarm around an egg until one of them unites with the egg. After the 

 conjugation the zygote develops into a new individual 



round cells into the water (see a, b, and c, Fig. 194). Other 

 swellings bear organs that throw out tiny cells, somewhat pear- 

 shaped, which carry two fine swimming threads, or cilia, on the 

 side (see d, Fig. 194). The smaller swimming cells cluster around 

 the large one until one of the small ones conjugates with the 

 large one. The zygote then divides and starts a new rockweed. 

 Where the two gametes are quite distinguishable, the conju- 

 gation is sometimes called fertilization (see section 35). 



347. Male and female. Where the two gametes are so unlike, 

 they are distinguished by special names. The large gamete is 



