No. 520] BASIS OF MEN DELIA N PHENOMENA 211 



terior of the body as in animals), doubtless act-omits for 

 the greater plasticity exhibited by plants. Indeed, it is 

 a matter for wonder that, under these conditions, plants 

 show such hereditary constancy. The accurate repro- 

 duction, generation after generation, of the most minute 

 hereditary differences, shows the relative fixity of the 

 material of the germ plasm. 



The work of the Marchals ('06 and '07) with the 

 mosses seems to show clearly that, during the meiotic 

 divisions of spore formation, an actual segregation of 

 sex-producing tendencies or elements takes place. Much 

 other evidence of a similar sort in plants might be cited 

 if time permitted, and the case of the sex chromosomes 

 in insects is too well known to require more than mention. 

 But while the reduction divisions seem the most natural 

 place to look for an explanation of the Mendelian propor- 

 tions, yet on the other hand there is much evidence that 

 the phenomena of "splitting" also occur at other times 

 in the life history. Bateson ( '05) has shown that in two 

 races of sweet peas {Lathyrus odoratus), one of which 

 has long pollen grains and the other round, the long 

 pollen character is dominant in the F v If a segregation 

 of characters took place here during the reduction divi- 

 sions, we should find 50 per cent, of each type of pollen 

 grain. But plants having long pollen give only long, or 

 they may give three plants with long pollen to one with 

 round. Only in "very rare and exceptional" cases is 

 there a mixture of long and round in the same plant, 

 and in such cases they are found only in certain flowers 

 of an individual. Rosenberg ('06) found that in the 

 hybrid Drosera longifolia X D. rotundifolia, in some 

 cases two pollen grains of a tetrad resemble each parent, 

 the grains differing chiefly in size. More recently, how- 

 ever, he has found ( '09) that the variations in the size 

 of the hybrid pollen grains probably depend upon the 

 numbers of chromosomes which enter the respective 

 daughter nuclei during reduction, and hence this differ- 

 ence in pollen grains will no longer serve as evidence for 

 a segregation of characters at this time. 



There is nothing to indicate that the phenomena of 



