l6 CHAP. III. RATE OF EVOLUTION OF OXYGEN 



The experimental water-plant found most suitable is, as 

 already stated, Hydrilla verticillata ; the advantages offered 

 by the plant are : (i) That it survives the winter months and 

 can therefore be used throughout the year ; the sensitive- 

 ness of this plant is, however, greater in spring than in 

 winter. (2) It yields a considerable amount of oxygen even 

 under a moderate intensity of light. (3) The slightly cuticu- 

 larised epidermis allows ready access of water containing 

 carbonic acid to the interior of the plant. (4) The large 

 intercellular spaces allow easy escape of the bubbles of 

 oxygen. (5) The cut end of the stem is not to any appreci- 

 able extent choked by mucilage ; and, finally, (6) there are 

 no complicating stomata to modify the actual activity. 



The Plant- Vessel 



This is a rectangular glass vessel having a capacity of 

 about200c.c. (fig. 2). It is filled with water free from nitrogen, 

 but containing the proper quantity of C0 2 in solution, which 

 is about 8 mg. per 100 c.c. Inasmuch as the physiological 

 condition of different plants can hardly be the same, it is 

 necessary that the complete investigation be carried out with 

 one and the same specimen of Hydrilla in normal condition. 

 A length of 10 cm. is in general taken from the upper part 

 of the plant bearing the most vigorously assimilating leaves. 

 The inverted specimen is placed in the centre of the vessel 

 and held in its place by the uppermost pair of leaves which 

 press slightly against the neck. The vessel is placed inside 

 a rectangular wooden box, not shown in the figure. This 

 box has an internal lining of non-conducting felt ; the plant 

 is thus protected from any sudden variation of temperature. 

 There is a hinged door in front, the opening of which exposes 

 the plant to light. For intermittent illumination a photo- 

 graphic shutter is substituted for the hinged door. Some 

 advantage is secured by fixing a mirror behind the glass 

 vessel, so that the direct and the reflected light produce a 

 more or less uniform illumination of the plant. 



