242 



MORPHOLOG Y. 



Fig. 3'9- 

 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 

 320 (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- 



ingnucleus 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 



are always twelve chromo- nuc j<y in , P oll , en . m ff her 



J c e 1 1 of podophyllum, 



sbmes so far as has been chromosomes at poles 



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



macrospore there are also twelve chromosomes. 



Fig. 320. 

 Second division 



Fig. 321. 

 Chromosomes uniting 

 at poles to form the 

 nuclei of the four spores. 

 (After Mottier.) 



