Carbon Dioxide in Maturation, etc., of Seeds. 411 



germination under ordinary and special circumstances we may be dealing not 

 with an insufficient oxygen stimulus but with an inhibitory cause or group of 

 causes. Such a condition might result in the case of the seed if the testa acts 

 in any way as limiting the aeration of the embryo, for we might expect then 

 two results : — 



(1) A reduction in the amount of oxygen reaching the embryo, and 



(2) A relative rise in the actual C0 2 pressure in the embryo tissues. 



The crucial question first arises, therefore, as to the actual effect of increased 

 pressures of C0 2 in the tissues of the embryo. The experiments which follow 

 have been immediately directed to ascertain in the first place the actual effect 

 of increased pressures of CO2 upon the germination of quickly germinating 

 seeds. 



Technique of Experiments made to Ascertain the Effect of Increased Partial 

 Pressures of C0 2 on Germination. — In setting these experiments a known 

 quantity of pure silica sand was first introduced into large flasks and 

 saturated with water. This was done by adding an excess of water and then 

 drawing it off by tipping the flasks. If this was carefully done the sand 

 Was left saturated with water in a layer adhering to the bottom of the 

 flasks. The seeds were carefully dropped on to this surface by means of a 

 glass tube, and, where necessary, as in the case of larger seeds, a further 

 measured quantity of H 2 was added. The flasks were then stoppered with 

 new rubber corks fitted with one glass tube closed by means of pressure 

 tubing and a pinchcock. Gases in any proportion desired were now quickly 

 introduced by first withdrawing a quantity of air by an air pump, the 

 amount being read by a pressure gauge. Where small quantities of C0 2 

 were desired up to 6 per cent, of an atmosphere, the operation was performed 

 by means of a specially made apparatus on the model of Hempel's gas 

 burette, using mercury. By means of this apparatus very accurately 

 measured amounts of air can be withdrawn and equal lots of C0 2 introduced. 

 For higher percentages of C0 2 the air pump was employed. The artificial 

 atmospheres were for the most part checked by analysis after setting. The 

 carbon dioxide, oxygen, and nitrogen employed were in all cases from 

 cylinders as supplied by the Carbonic Acid Company and British Oxygen 

 Company. 



