160 



GENERAL BOTANY 



the offspring resulting from the sexual process to produce new 

 plants which are either pleasing on account of their beauty or 

 are useful for food, forage, or fuel. 



Other advantages have been attributed to the sexual process 

 besides those which accrue to a species by the production of a 

 varied offspring, but these are as yet unproved. 



In the higher plants the male gametes are produced in the so- 

 called pollen tube, which is an outgrowth from the pollen grain. 

 They are small, naked cells with a conspicuous nucleus and a 



Egg cell 



Seedling plant 

 FIG. 83. Sexual reproduction and doubling of the chromosomes in fertilization 



very thin sheath of cytoplasm. The end of the pollen tube rup- 

 tures when it reaches the vicinity of the egg in the ovule, and 

 frees the male gametes. The female gamete is larger than the 

 male gamete and is furnished with a more conspicuous nucleus 

 and a larger amount of cytoplasm. 



When the two naked gamete cells come together, the male 

 and female nuclei, called pronuclei, approach each other and 

 finally unite to form a new double nucleus, the conjugate 

 nucleus, or fusion nucleus. This process of fusion of male and 

 female sex cells is called fertilization (Fig. 83). 



The zygote which results is a double cell structurally and 

 functionally ; hence the embryo and the young plantlet in the 

 seed must possess all of the characters of the parents which 

 entered into the zygote cell through the gametes. Since the male 

 gametes are produced in the pollen grains and the female gamete 

 is deeply buried in the ovule of a higher plant, a complicated 



