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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