376 Report oF THE HortTICULTURAL DEPARTMENT OF THE 
SPECIFIC GRAVITY AS RELATED TO RIPENESS. 
If the seeds compared are of unequal degrees of ripeness, ob- 
viously differences in chemical composition exist. Nowacki 
found that the specific gravity of wheat decreases from the stage 
of milk ripeness to that of dead ripeness, and Wollny found the 
same to be the case with rye. But with peas Wollny found just 
the opposite changes in specific gravity as related to ripening. 
The writer has not himself yet investigated the relation between 
degree of ripeness and specific gravity; but theoretically such a 
correlation may be assumed to exist, not merely on account of 
differences in composition but on account of the greater shrinkage 
which unripe seeds undergo in drying out. Wollny has called 
attention to this point, as has just been noted, with reference to 
certain seeds in which the seed coat adheres closely or quite so 
to the parts within; but Nobbe also calls attention to differences 
in specific gravity induced in seeds with a rigid seed coat. In 
this case the endosperm or cotyledon may become slightly reduced 
in volume in drying out; but the seed coat remains inflexible. 
If such a change as this actually takes place, the endosperm or 
cotyledon within would shrink away from the seed coat, leaving 
a hollow region between. Now just such a hollow region is found 
in seeds with a rigid seed coat such as grapes, squash, and various 
other seeds. Whether, however, the seed coat in any of these 
seeds is completely filled in the unripe seed, or in the seed as it - 
comes fresh from inclosing moist tissues, the writer has not yet 
made the examinations to determine. If, however, the fleshy 
parts of seeds do change in volume unequally at different stages 
of ripeness, it appears probable that selection of seed by specific 
eravity would be really a selection according to degree of 
ripeness. 
SPECIFIC GRAVITY AS RELATED TO STRUCTURE OF 
SEED. 
Differences in composition are not sufficient to explain the 
observed differences in specific gravity of seeds. These differ- 
ences, then, must be sought in differences in structure. In fact, 
it must be borne in mind that seeds are by no means homogen- 
eous, but are rather porous and are unequally developed in dif- 
