614 Mr. F. Kidd. The Controlling Influence of 



It will be seen that the amount of oxygen present has a definite effect 

 upon inhibition by CO2. Where there is only a pressure of 5 per cent, 

 oxygen, complete inhibition is obtained by 15 per cent. CO2, but, with 

 30 per cent, oxygen present, as much as 30 per cent. CO2 is scarcely 

 sufficient at the temperature used to cause inhibition. 



The result of these experiments, therefore, indicates that a rise in the 

 partial pressure of oxygen within the limits experimented on necessitates an 

 increase in the amount of C0 2 necessary to produce inhibition in the seeds of 

 Brassica alba. Conversely, a fall in the partial pressure of oxygen reduces 

 the necessary amount of C0 2 to cause inhibition. 



Section III. — Carbon Dioxide as a Factor in the Dormancy of the Maturing 



Seed on the Plant. 



(a) Arrested Development of Maturing Seeds not due to Lack of Moisture — 

 Retarding Influence of the Testa. — The maturation of the seed iu normal 

 conditions has certain features upon which it is desirable to dwell briefly. 



The growth of the embryo proceeds continuously after fertilisation. In 

 some cases it quickly reaches an advanced stage, and the radicle, plumule, 

 and cotyledons may be formed very early. This growth, moreover, appears 

 to resemble in some respects the growth which takes place subsequently, 

 after germination, but in others it has the appearance of partial inhibition, 

 the radicle apparently being not free to sprout as in germination. This 

 appearance of inhibition increases in the cases of most seeds, until at the 

 stage of complete maturation growth is apparently arrested or suspended. 

 That there is some restraining cause tending to prevent growth present in 

 the seed during the series of changes which is producing maturation may be 

 proved, as in the experiments following, by the fact that the embryo, often at 

 a comparatively early stage, though the seed be far from ripe, can be caused 

 to sprout if removed to air. 



The following experiments were conducted in order to show that neither 

 lack of water nor any physiological insufficiency in the embryo can be 

 considered as the cause preventing the still maturing embryos of beans and 

 peas from sprouting, and so becoming cases of viviparity : — 



(1) Two lots, 10 peas and 10 beans, were taken from pods which were still 

 perfectly green and hardly yet fully swelled. These two lots were set to 

 germinate at 20° C. on damp sand, with the result that all the seeds 

 germinated perfectly. From these experiments it is clear that, in the case 

 of the bean (Vicia faba) and of the pea (Pisum sativum), for some con- 

 siderable period before the natural drying process commences, and while the 

 growth of the pods is continuing, the seeds, if removed and placed in 



