1894.] on Vegetable Assimilation and Respiration. 167 



In an experiment on the leaf of Ampelopsis hederacea, with, no 

 stomata on the npper surface, air containing 1'6 per cent. C0 2 was 

 supplied to both surfaces at the rate of 22 c.c. for every fifteen 

 minutes ; of the 0'37 c.c. of C0 2 thus entering the capsule on the 

 lower surface in this time, 0*14 c.c. was absorbed, while none at all 

 was absorbed by the upper surface. With a leaf of AUsma, on the 

 contrary, the whole of the C0 2 O15 c.c. supplied in fifteen minutes 

 to the upper surface was absorbed, and Oil c.c. of that supplied to the 

 lower. In this leaf the stomata are in the ratio of 135 above to 100 

 below, to which ratio the absorption numbers closely correspond. A 

 very simple experiment will show that stomata are practically the sole 

 path of entry of C0 2 for assimilation. If part of the lower stomatic 

 surface of any leaf with no stomata on its upper surface (Sparrmannia 

 gives very clear results) be coated with wax so as to mechanically 

 block the stomata, no starch can be formed in that area, while the 

 adjacent areas become rich in starch. I performed this experiment in 

 1893, and showed it to some botanists ; recently it has been published 

 by Stahl.* 



As stated previously, the theory of " cuticular exchange " has 

 hitherto found its strongest support in the experiments of Boussin- 

 gault, in which, under similar conditions, leaves of Nerium Oleander 

 assimilated less when the upper astomatiferous surface had been 

 coated with an unguent than when the lower stomatiferous surface 

 had been so coated. From this he drew the obvious conclusion that 

 the C0 2 of assimilation normally passes into the leaf through the 

 cuticle of the upper surface. Exposure of the interesting experi- 

 mental fallacy here concealed, however, quite reverses the interpre- 

 tation of these experiments. Boussingault experimented with leaves 

 in an atmosphere containing 30 per cent. CO 2 . Now the optimum 

 percentage of C0 2 for assimilation is very low for this leaf, and the 

 real interpretation of the result is that the diminished decomposition 

 of C0 2 in the leaf with open stomata is due to its obtaining not less 

 C0 2 but more C0 2 . In fact, there penetrates into it so much C0 2 that 

 its assimilatory activity is lessened, and falls below that of the other 

 leaf into which, owing to the blocking of the stomata, the C0 2 only 

 diffuses very slowly, and cannot exceed the optimal strength. This 

 view has been conclusively proved by a series of experiments in 

 different strengths of CO 2 . In a small percentage the leaf with its 

 stomata open decomposes more C0 2 than the leaf with its stomata 

 blocked a result just the reverse of Boussingault's. 



Further evidence on the possible paths of gaseous exchange has 



been obtained by investigating the degree to which diffusion of. CO 2 



can be artificially produced through the living leaf. Strong mixtures 



of C0 2 are led continuously across one surface of a leaf, and the 



* ' Botanische Zeitung,' July, 1894. 



