34 Mutations and Evolution. 



When cell walls are formed later a hexad of pollen grains is 

 frequently produced, four large ones containing seven chromosomes 

 and two small containing one or two chromosomes each, it is 

 doubtful if any of these pollen grains are functional regular 

 meiosis takes place occasionally hut it is interesting that a pollen 

 grain with only one chromosome, though diminutive, can still form 

 its exine covering in an apparently normal manner. And such 

 irregularities always make it possible that an extra chromosome 

 may find its way into one of the daughter nuclei, as in the origin of 

 (E. I atd. 



