26 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 water 1 , 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 

 following day. 



(33) The importance of the stomata in supplying the 

 path for gaseous exchange 2 . 



For this experiment leaves should be employed in 

 which the stomata are all on the lower surface. Stahl 

 uses Primus padus; we find Sparmannia africana gives 

 good results, and no doubt many other plants would 

 answer the purpose. The lower surface of one half of a 

 leaf is carefully painted with vaseline, or as Stahl re- 

 commends with melted cocoa-fat and beeswax. The 

 plant having been exposed to a good light for two days, 

 the leaf is subjected to the iodine test. The painted 

 half (in which the stomata are blocked) will be either 

 quite or nearly starchless, while the control half shows a 

 normal amount of starch. 



(34) Effect of excess of CO*. 



To show that excess of CO 2 diminishes assimilation 3 

 floating water-plants are convenient. We use Callitriche, 

 and possibly Lemna might be used, but these must be 



1 Nagarnatz (Sachs' Arbeiten, in.) shows that leaves covered with 

 bloom can assimilate under water. 



2 Stahl, Botan. Zeltung, 1894. See also F. F. Blackman, Phil. Trans. 

 1895, and in Science Progress, 1895. 



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



