CHAPTER III 



DETERMINATION OF RATE OF EVOLUTION OF EQUAL VOLUMES 



OF OXYGEN 



Frequency and volume of the bubblets — Effect of depth of immersion — 

 Effect of choking of the cut end by mucilage — Spontaneous variation 

 in the frequency of the bubblets — Determination of successive evolu- 

 tion of equal volumes of oxygen — The plant-vessel — The Bubbler — 

 Measurement of photosynthetic activity — Determination of the con- 

 stant of the Bubbler. 



The most important factor in the accurate estimation of 

 photosynthetic activity is the measurement of the rate of 

 evolution of the oxygen. The usual method of estimation 

 by counting the bubblets given out by the plant is, however, 

 not trustworthy for the following reasons : 



(i) The bubblets from an intact water-plant force their 

 way out from the margin of the leaves in a very irregular 

 manner ; this difficulty may be removed by making a large 

 opening by a transverse cut, when the gas-bubbles come 

 out of the section instead of the side of the leaves. But the 

 bubblets are often given off so rapidly that accurate count- 

 ing of them becomes impossible. In less active specimens, 

 when the successive intervals are longer, the counting of 

 the bubblets gives no accurate indication of the photo- 

 synthetic activity, since their volume does not remain 

 constant. 



(2) The frequency and volume of the bubblets depend on 

 the depth of the immersion of the cut end below the level 

 of the water. If the depth be in any way increased the 

 frequency becomes decreased while the volume undergoes 

 an increase. 



(3) Even the maintenance of a constant depth of immer- 

 sion does not ensure uniformity in the rate of bubbling, for 



