THE TWO GENERATIONS 



413 



the fertilized egg is double that of the sperm or egg. It follows 

 that, unless the number of chromosomes is reduced somewhere 

 in the life cycle of the plant, each generation of plants would have 

 double the number of chromosomes of the preceding generation. 

 This doubling of the chromosome number in each generation 



e 



FIG. 368. Diagrams showing the difference between ordinary cell divi- 

 sion and the reduction division. To make the diagrams easy to follow only 

 two chromosomes in each case are represented, but their behavior is typical 

 of all the chromosomes of the nucleus. One chromosome has been blackened 

 and the other left white to indicate that they differ in that one consists of 

 chromatin material of the father parent and the other, of the mother parent 

 of the individual whose cell division is illustrated by the diagrams. The 

 upper diagram illustrates the behavior of chromosomes in ordinary cell divi- 

 sion, showing the chromosomes at a soon after organization, their arrange- 

 ment on the spindle fibers and splitting lengthwise at b, the separation of the 

 longitudinal halves at c, and the formation of the new nuclei at d, with each 

 new nucleus containing a longitudinal half of each of the original chromo- 

 somes. In the lower diagram, illustrating the behavior of chromosomes 

 in the reduction division, the chromosomes are paired at e, arranged on the 

 spindle in pairs at /, separated as whole chromosomes at g, and thus each 

 nucleus at h receives one chromosome of the pair and not half of each chro- 

 mosome as in ordinary cell division. 



would soon result in a disastrous piling up of chromosomes. In- 

 vestigations show that the sporophyte has twice the number of 

 chromosomes of the gametophyte, but that the spores formed by 

 the sporophyte have the gametophytic number. The transition 

 is made in the mother cells, that is, in the cells which form the 

 spores, and by these cells dividing in such a way that the chromo- 



