82 GENERAL BOTANY 



of the tetrad then becomes a spore, from which the gamete 

 cells, egg and sperm, are ultimately derived. These spores and 

 the gametes derived from them will consequently have the 

 reduced number of chromosomes characteristic of the reproduc- 

 tive cells of the higher plants. 



Fig. 44 illustrates the relation of the reduction division to 

 sexual reproduction and the development of a plant organism. 

 In the figure the male and female gametes, which have been 

 derived from the spores with the reduced number of chromo- 

 somes, are represented as having three chromosomes each. 

 When these unite in fertilization, a zygote cell is formed 

 which contains six chromosomes, or the sum of the chromo- 

 somes of the male and female gametes. The divisions of the 

 zygote cell which follow in the development of the embryo 

 and seedling are of the vegetative type represented in Fig. 43, a, 

 and hence each cell of the seedling will be furnished with six 

 chromosomes. This number remains constant also in all the 

 vegetative cell divisions which occur during the development 

 of the adult plant. 



It is now known that every species among the higher plants 

 has the same chromosome history as that sketched above, in 

 that the chromosomes are always doubled when fertilization 

 occurs and are reduced to one half the vegetative number in 

 the spores and the gametes. A similar reduction division occurs 

 in animals during the formation of the egg and sperm cells. It 

 is probable also that all of the lower plants have a reduction 

 division at some point in their life history which corresponds to 

 that described above for the higher plants. It is easily seen that 

 if reduction in the number of chromosomes did not occur at some 

 point in the life cycle of each individual organism, the chromo- 

 somes would ultimately become innumerable in the cells of all of 

 the higher plants. Reduction division is also supposed to have 

 an important bearing on the method of inheritance of parental 

 characters, which will be discussed in a later chapter. The great 

 precision and regularity with which the vegetative and reduction 

 cell divisions are carried out in the life of each organism is a 

 sufficient guaranty of their fundamental biological importance. 



