442 
Kidd and West . — The Role of the Seed-coat in 
turns yellow, the effect of the presence of the seed-coats is markedly 
reduced. The presence of the seed-coats in the yellow-ripe stage only 
causes dormancy or delayed germination. Finally, when the testa is com- 
pletely yellow and the seed is dry, the testa no longer has any appreciable 
effect upon germination. 
Since the embryos will germinate freely at all three stages if the seed- 
coats are removed, there does not seem to be any ground for seeking an 
explanation of the above differences in behaviour between green-ripe, 
yellow-ripe, and dry-ripe seeds in progressive changes occurring in the 
embryo itself. 
The progressive changes which occur in the testa, to which we must 
therefore look for an explanation, may be described as follows. In the 
green-ripe seed the embryo is enclosed in a relatively thick green coat con- 
sisting of actively functioning tissue, while between this coat and the embryo 
a considerable quantity of liquid is usually found. In the yellow-ripe seed 
the testa is relatively thinner and presumably less active, but is still living. 
The liquid between the embryo and the seed-coats has by this time 
disappeared. In the dry-ripe seed the testa is a thin membrane and 
presumably dead. 1 
Considered as an obstacle to the gaseous exchange of the embryo the 
testa probably behaves in relation to its thickness in the same way as a film 
of water ; in other words, the thicker the testa the more slowly do gases pass 
through. 
It is a well-known fact that immersion in water does inhibit the germi- 
nation of most seeds, and that prolonged immersion is fatal (see Kidd and 
West, 11 and 13 2 ). 
Further, the living testa, i. e. the testa in the green-ripe and yellow-ripe 
stages as opposed to the dry-ripe stage, will not only consume oxygen and, 
as it were, steal it on its way to the embryo, but on account of its production 
of carbon dioxide will also tend to hinder the escape of this gas from the 
tissues of the embryo. The progressive changes in the testa described above 
will therefore for two reasons be in favour of a progressively greater con- 
centration of oxygen available for the embryo, and a more rapid escape of 
carbon dioxide. 
In this connexion it is interesting to note that Demoussy (5) found that 
hydrogen peroxide increased the percentage of germination of old Cress seeds. 
Hydrogen peroxide has a lethal action upon the saprophytic flora of the 
dead seed-coats. He suggested that non-germination in these old seeds was 
to some extent due to the respiratory action of moulds or bacteria present in 
1 For details as to the histological and microchemical characters of the testa in the case of the 
genus Brassica , the reader is referred to papers by Schroeder (21), Sempelowski (22); Holfert (8), 
Burchard (3), Gram (7), Kinzel (14), Pieters and Charles (20), Kondo (16), and Kidd and West (10). 
2 In this paper the literature dealing with the effect of soaking seeds in water is critically 
reviewed, 
