THE PROBLEM OF SEX IN PLANTS . 353 



320. Maleness and Femaleness. — Spirogyra. In nearly 

 all cases of cell-fusion it is possible to recognize some 

 difference, either of structure or of behavior, between the 

 gametes. In Spirogyra, for example, it has long been 

 noticed that if one of the cells of a filament passes over to 

 the other filament through the conjugation-tube, all the 

 cells of that filament will ordinarily do the same. Thus 

 after conjugation is over all the cells in one filament will 

 be found empty, while all the cells of the adjacent, con- 

 jugating filament will contain zygotes. This behavior, 

 however, varies under certain conditions and with differ- 

 ent species. 



Recent studies by York have revealed the fact that the 

 supplying gamete of Spirogyra always possesses less starch 

 and a less number of starch-formers (pyrenoids) than does 

 the receiving gamete (Fig. 263). It has also been noted 

 that the supplying filaments (male ?) are less vigorous, 

 vegetatively, than the receiving filaments (female ?). 



321. Sexuality in Molds.— One of the most interesting 

 of recent discoveries in connection with sex in plants, is 

 that of the existence of two strains of different sexual value 

 in the molds. It was known for a long time that conjuga- 

 tion and the formation of zygotes in these plants could not 

 always be secured when desired; that is, conjugation would 

 not occur between every two individuals. At first it was 

 thought that the explanation lay in the fact that the ex- 

 ternal conditions (temperature, light, moisture), might 

 not be just right, or that the two adjacent plants were 

 not in the right condition as to age, or nutrition, or 

 otherwise. Finally, as stated in Chapter XIX, Blakeslee, 

 by careful experimental studies, found that there are two 

 unlike strains of many of the molds, and that whenever a 



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