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 (Hig. 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 condi- 
