26 EVOLUTION OF PLANTS 



in lizards the renewal of the tail, but, owing to the 

 great complexity of the higher animals, they are unable, 

 except in rare instances, to produce new individuals 

 except from eggs. With plants the case is different, 

 and even among the highest ones some forms of non- 

 sexual multiplication by budding is almost universal. 

 This is no doubt largely due to the much lower degree 

 of specialization in the tissues of even the highest plants. 



The first evidence of sex is manifest very early among 

 both animals and plants. Sexual reproduction consists 

 essentially in the formation of a germ by the union of 

 two cells which fuse completely into one. In many 

 unicellular plants, such as the desmids (Fig. 1, B) and 

 lower green monads (Fig. 6, F), there is no apparent 

 difference between the sexual and non-sexual cells, but 

 two individuals fuse into one, or at least the protoplasm 

 and nuclei of the two cells fuse, the resulting cell then, 

 as a rule, secreting a new wall about itself, and either 

 forming a new plant at once, or by division giving rise 

 to two or more new plants. In these lowest forms the 

 two uniting cells are entirely similar, and we cannot 

 speak of male and female cells. 



The first indication of the separation of the sexes is 

 seen in the formation of sexual cells or gametes, of un- 

 equal size (Fig. 6, F). These cells are usually motile, 

 being provided with cilia, and resemble exactly the non- 

 sexual swarm-spores, except that they are incapable of 

 germinating unless two of them unite to form the 

 "zygote," or germ of the new plant. The larger of 

 the gametes is the female, the smaller the male cell. It 

 is interesting to note that in some of the lowest forms 

 where gametes occur, these may under certain cpndi- 



