CH. Il] GAS EVOLVED. 35 



water. The relative positions of starch-former and starch- 

 grain and the elongated crystalloid are well shown in 

 Strasburger's figure 29. The leucoplasts in the rhizome of 

 /m gerinanica are given in his fig. 80. 



Section B. The Evolution of Oxyg^en. 



(41) Bubbles of gas given off. 



Place a branch or two of a submerged water-plant, 

 such as Hottonia, Potamogeton crispus, or Elodea, in a 

 beaker filled with spring water which has been in the 

 laboratory for 12 — 24 hours, and has acquii^ed the temper- 

 ature of the room. The cut ends of the plants must be 

 upwards ; and must be below the surface, to effect which 

 it may be necessary to tie the specimens to a glass rod 

 (see Pfefifer, Physiologie, I. fig. 17, and Detmer, fig. 12). 

 The beaker is to be placed in sunlight, and evolution of 

 gas from the cut ends of the specimens to be observed. 

 To obtain a convenient series of small bubbles Pfeffer 

 recommends varnishing the cut end of the shoot and 

 pricking a fine hole in the membrane so produced. Select 

 a branch which seems to be yielding a satisfactory 

 amount of gas, and record, with a stop-watch, the time 

 which elapses while 10 or 20 bubbles are given off. The 

 observation must be repeated until the rate of bubbling 

 is fairly constant. It is important to know that the 

 evolution of bubbles of gas may be produced by other 

 causes than illumination. Thus a plant which is exposed 

 to feeble illumination and is not giving off bubbles 

 may be made to do so by being transferred to a beaker 



3—2 



