278 BOTANICAL GAZETTE [OCTOBER 
temperature of 19°. Since the difference of the two seed coats in 
the matter of oxygen diffusion is rather slight, it is not remarkable 
that the structural difference is not radical. This slight difference, 
however, is sufficient to raise the minimum germinative temperature 
and secure the delay of the upper seed. 
Growth of upper seeds—It is now obvious how the delay of the 
upper seeds is secured. But why do they grow at all in nature? 
This comes about by a partial disintegration of the seed coats, which 
is clearly shown by a change in appearance from shiny brown to a 
dull black or in some cases to colorless, and results in the admission 
of more oxygen. The length of the delay depends upon the ability 
of the seed coat protected by the surrounding bur to resist the factors 
of disintegration in the soil. The portion of the bur covering the 
lower seed decays within a few months after burial, while the portion 
covering the upper seed is always far more persistent. A variation 
in the persistence of the portion of the bur covering the upper ‘seed, 
as well as the variation in the ability of the seed coat to resist the 
factors of disintegration independent of the bur, gives considerable 
variation in the length of delay of the upper seed. These facts 
show why only a few of the upper seeds grow the first year after 
ripening, the vast majority the second year, and a few not until the 
third and fourth year. 
Table VII shows the effect of a period in the ground upon the 
seed coats and upon the vitality of the embryos of the upper seeds of 
X,._canadense. Burs produced in 1904 were gathered in November 
of that year and stored in the laboratory until March 1905. At this 
time half these burs were buried and the other half kept in the labora- 
tory. In November 1905 the upper seeds of 1904 burs stored in the 
laboratory, of 1904 burs buried eight months, and of 1905 burs 
gathered from the same patch, were removed and put to germinate. 
At 28-33°, with coats intact as Table VII shows, upper seeds of 1904 
buried eight months gave 96 per cent, germination, upper seeds of 
1904 stored in the laboratory gave o per cent., and upper seeds of 
1905 gave 3 per cent. As shown in the same table, similar seeds 
with the coats removed in a germinator at 18-22° (near the minimum 
germinative temperature with the seed coats broken) gave in UpPtT 
seeds of 1905 prompt germination, in upper seeds of 1904 stored 
