52 



Reproduction and Meiosis 



pyle, a small hole in the ovule, and enters the embryo sac. The 

 tube nucleus disintegrates, while one sperm nucleus fertilizes 

 the egg to form the zygote and the other unites with the two 

 nuclei at the center of the embryo sac, the polar nuclei, to form 

 endosperm, sl tissue in which food is stored for the developing 

 embryo. The zygote is the first stage of the new sporophyte. 



Higher plants have a much more com- 

 plicated life cycle than animals because 

 two generations are necessary to com- 

 plete the entire cycle. 



Meiosis 



The basis of the difference between 

 the two meiotic divisions and any two 

 successive somatic mitoses is to be found 

 at the beginning of the prophase of the 

 first of the two meiotic divisions. It has 

 been pointed out that the chromosome in 

 the resting stage is either a single struc- 

 ture or is composed of two chromone- 

 mata which are in such an intimate re- 

 lationship that they behave as a single 

 structure. By the beginning of prophase 

 in an ordinary somatic mitosis, either the 

 single chromonema has doubled or the 

 two chromonemata of each chromosome 

 have become sufficiently separated that 

 the chromosomes are definitely double 

 structures. At the prophase of the first meiotic mitosis, how- 

 ever, the chromosome is still effectively single, just as it was in 

 the resting stage. This is a fundamental difference between 

 mitosis and the first meiotic division. 



Another important difference is that shortly after the chromo- 

 somes appear, they begin to lie alongside one another in pairs. 

 Each chromosome pairs with the chromosome which is identical 

 with it in size and shape — in other words, with its homologue. 

 In meiosis, it is not until ajter the chromosomes have paired 

 that each chromosome becomes a double structure. After each 

 chromosome has become double, a chromatid from one homo- 



FiG. 15. Pollen tube 

 showing two sperm nu- 

 clei. (Courtesy of Dr. 

 George H. Conant.) 



