22 ASSIMILATION. [CH. II 



(32) Terrestrial leaves under water. 



To show that the leaves of land-plants do not form 

 starch as those of aquatic plants do under watery it 

 is only necessary to tie a leaf so that it is partly immersed 

 in a beaker of water. The experiment may be started in 

 the morning and concluded on the afternoon of the follow- 

 ing day, 



(33) Effect of excess of GO,. 



To show that excess of CO^ diminishes assimilation ' 

 floating water-plants are convenient. We use Callitriche, 

 and possibly Lemna might be used, but these must be 

 kept a long time in the dark before they are de- 

 starched. Two graduated jars of 200 c.c. capacity are filled 

 with and inverted over water, and plants of Callitriche, 

 which have been previously deprived af starch, are passed 

 under the edge and allowed to float up. Into one jar 

 equal quantities of air and CO;,, while into the other 

 12 volumes of air to one of CO, are passed. The propor- 

 tion of GO, in the atmospheres so prepared does not of 

 course remain constant, since the water absorbs the gas. 

 But if the experiment is started in the evening and 

 concluded in the evening of the next day, one jar will 

 certainly contain far more than the optimum of CO,, 

 while the other will not fall much below the optimum. 

 A still simpler plan is to use beakers of about 800 c.c. 

 capacity inverted in saucers of water. The beakers are 



^ Nagamatz (Sachs' Arbeiten, iii. ) shows that leaves covered with 

 bloom can assimilate under water. 



' Godlewski. Sachs' Arbeiten, i. p. 343. 



