128 THE THEORY OF THE GENE 



give ratios different from those in the normal plants. As 

 yet little has been done in this direction, although Wett- 

 stein has found clear evidence of genetic inheritance in a 

 few species crosses of mosses, and Allen, in the related 

 group of liverworts, has genetic evidence for two charac- 

 ters of the gametophyte. 



In those mosses with separate sexes and in certain 

 liverworts it has been shown by the Marchals, by Allen, 

 by Schmidt, and by Wettstein that the sex-determining 

 elements are sorted out at the time of spore formation. 

 An account of these observations and experiments will 

 be given in the chapter on sex. 



There are many important questions for embryology 

 rather than for genetics relating to the size of the cells 

 of tetraploids. In general it may be said that the cells 

 are larger, and frequently twice as large, but there is a 

 good deal of variation in the different tissues in these 

 respects. 



The size of the whole plant as well as some of the other 

 peculiarities of the tetraploid are due apparently to the 

 increase in size of its cells. If this is correct, it means 

 that these characteristics are developmental rather than 

 genetic. The way in which tetraploids arise has to some 

 extent been already considered. The methods that have 

 been suggested, as to how the increase in the amount of 

 cytoplasm in the cells of the tetraploids takes place, call 

 for further examination. 



If two cells in the germ-track should fuse, and their 

 nuclei then or later unite, a tetraploid cell might result. 

 If the double cell continued to maintain a double volume 

 in the growth period, an egg of twice the normal size 

 would be expected to result. The number of cells of the 

 larger embryo would also be expected to be the same as 

 that characteristic of the normal embryo. 



There is, however, another possibility, namely, that the 



