Notes . 
167 
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 ( Sparmanma 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 1 . 
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. C0 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 C0 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 C0 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 
amount which diffuses through is estimated. On supplying 31 per 
cent. C0 2 to the lower surface of a leaf of Nerium only 0:035 per 
cent, appears by diffusion in a slow current of air kept continually 
1 Botanische Zeitung, July, 1894. 
