KEDUCTION AND SEGREGATION 



Uala •.lUotl C^) ^ 



343 



Megaspore 

 Mature CeU 



Fig. 95. — The Life-Cycles of Plants and Animals Compared. 

 The diagrams are largely self-explanatory and show that the animal 

 and higher plant both originate iDy the union of a sperm and an egg, 

 thereby doubling the chromosome's number so that each cell has n 

 pairs of chromosomes. When the animal forms its gametes the mem- 

 bers of each pair separate from one another and go to opposite poles 

 of the spindle. This results in reducing the number by half and 

 further provides that each gamete has but one chromosome of each 

 pair. Fertilization again unites a sperm and an egg and doubles 

 the number again. In the plant the history is the same in so far 

 as the cycle up to reduction is concerned. The reduction divisions 

 give rise to spores instead of gametes, and spores develop into micro- 

 scopic gametophyte or sexual plants in the higher plants which in 

 turn produce the gametes. One sperm fuses with the egg to produce 

 the new plant and the other in many plants unites with the endosperm 

 nucleus to produce the endosperm, a part of the seed peculiar to 

 certain flowering plants such as com. 



