Recent Advances in the Study of Heredity. 281 
may be reached indirectly, by measuring the converse difference, 
namely that between the amount of water which a dry round Pea 
absorbs when it germinates and that absorbed by a dry wrinkled 
Pea under similar conditions. The result of such a determination 
is to show that a wrinkled Pea takes up very much more water in 
proportion to its weight when dry than a round one does. The 
method which I employed in this determination was to weigh the 
Pea dry, immerse it in tap water kept at as constant a temperature 
as possible and weigh it again after the lapse of twenty-four hours. 
The difference between the two weights of course gives the amount 
of water absorbed ; and I propose to regard the ratio, expressed as 
a percentage, between this amount and the weight of the dry Pea 
as an index of the absorptive capacity of the Pea. Determined in 
this way the average absorptive capacity of some round Peas was 
86%, and that of some wrinkled ones was 120%. It will he seen 
that whilst a round Pea does not absorb its own weight of water 
during twenty-four hours, a wrinkled one absorbs considerably more 
than its own weight of water. From this we are, 1 think, justified 
in concluding that the converse of the process which we have thus 
measured also takes place, namely that a wrinkled Pea gives up 
more water during ripening than a round one does. 
Now, it is known that the reserve material in the cotyledons of 
Pisum first appears in the form of sugar, and that this is gradually 
converted into starch, which exists in the form of grains. It is 
further known that water can escape more easily from a sugary 
solution than it can from a starch grain ; it is, so to speak, “locked 
up” more effectively in the latter than in the former. These two 
facts naturally lead one to suppose that the reason that more 
water escapes from the wrinkled Pea during ripening than from the 
round is that the water is only held by sugar in the case of the 
wrinkled and by starch in the case of the round. This is broadly 
speaking true: it is, however, not the case that there is no starch 
in the wrinkled Pea. But there are many facts which go to shew 
that there is a great deal more sugar in the wrinkled Pea than in 
round. In the first place the profound difference between the 
structure of the starch grains of the two types (see Figs. 1 and 2) 
suggests a fundamental difference between the chemical nature of 
the reserve materials of the two. In sections of the cotyledons the 
starch grains of wrinkled Peas can be seen to occupy considerably 
less space in the cells than the grains in round ones do : in the 
former there is no inconsiderable amount of interstitial material, 
