GAMETOPHYTE AND SPOROPHYTE. 



343 



Fig. 408. 



Different stages in the separation of divided 

 U-shaped chromosomes at the nuclear plate. (After 

 Mottier ) In podophyllum. 



while the other chromosome of each pair goes to the opposite 

 pole. The chromosomes here unite to form the daughter nuclei. 



Each of these nuclei now 

 divide as shown in figure 

 409 (whether the chromo- 

 somes in this second divi- 

 sion in the mother cell split 

 longitudinally or divide 

 transversely has not been 

 definitely settled), and four 

 nuclei are formed in the 

 pollen mother cell. The 

 protoplasm about each one of these four nuclei now surrounds 

 itself with a wall and the spores are formed. 



The number of chromosomes usually the same in a given 

 species throughout one phase of the plant. In those plants 

 which have been carefully studied, the number of chromosomes 

 in the dividing nucleus has been found to be fairly constant in a 

 given species, through all the divisions in that stage or phase 

 of the plant, especially in the embryonic, or young growing 

 parts. For example, in the 

 prothallium, or gameto- 

 phyte, of certain ferns, as 

 osmunda, the number of 

 chromosomes in the divid- 

 ing nucleus is always twelve. 

 So in the development of 

 the pollen of lilium from 

 the mother cells, and in the 

 divisions of the antherid 

 cell to form the generative 

 cells or sperm cells, there 



Fig. 409. 

 Second division 



of 



twplvp rhrnmn- nuclei in pollen mother at 

 twelve D cell of F podoph yii um> 



Fig. 410. 



Chromosomes uniting 

 poles to form the 

 nuclei of the four spores. 

 (After Mottier.) 



SOmeS SO far as has been chromosomes at poes 



found. In the development of the egg of lilium from the 

 macrospore there are also twelve chromosomes. 



