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THE AMERICAN NATURALIST [Vol. XLII 



somes, the sporophyte taking its peculiarities because of the 

 doubling of the number which results from the sexual fusion of 

 gamete nuclei, and giving up these characteristics when the 

 number of chromosomes are reduced at the end of the sporo- 

 phytic phase. This view that nuclear structure and more par- 

 ticularly the number of chromosomes -ives the physical basis 

 for alternation of generations was originally stated by Stras- 

 burger and has received support from a large amount of research 

 on life histories throughout the plant kingdom. It has in the 

 opinion of some authors reached the si a ire worthy of statement 

 as a law of development, as indicated by the expression x and 2x 

 generations applied to gametophyies and sporophytes. 



However, the cytological investigation of apogamy in the 

 seed plants as well as in the ferns has shown for a considerable 

 number and wide range of forms thai the gametophyte genera- 

 tion may have the sporophytic number of chromosomes, and now 

 in Xephrodium then 1 is estaUished the first instance in which 

 a sporophytic generation may develop with the gametophytic 

 number. 



This evidence may he regarded by some as cutting at the roots 

 of the antithetic theory of alternation of general ions, but this 

 does not follow. It is (dear that an increase or decrease in the 

 number of chromosomes within a certain range does not affect 

 the morphology of the phase of the plant's life history con- 

 cerned, and the cause of the specific characters of gametophyte 

 and sporophyte must rest upon other factors. What these may 

 be is problematical; it is not unlikely that a variety of factors 

 is concerned. It is probable that the peculiarities of every 

 species demand at least a certain amount of chromatin with a 

 specific composition, but there is no reason to assume that this 

 must be contained in a fixed number of chromosomes, and fur- 

 thermore multiples of the minimum amount required would not 

 be expected to introduce new characteristics except as it might 

 give increased vigor or vitality. Then there is the cytoplasm 

 to be considered and perhaps of even greater importance the 

 complex reciprocal relations that must exist between the nucleus 

 and cytoplasm. 



Bradley M. Davis. 



