( 663 ) 
be readily distributed through the intercellular spaces by diffusion, 
but that this distribution can be limited by the wholly or partially 
closed tissue of the veins. In that case one would come to the con- 
clusion, that such leaves are divided up by smaller and larger veins 
into areas, within which carbon dioxide transportation can readily 
take place. The passage from any one such area to another is 
difficult however, or quite impossible. The distance across which 
carbon dioxide can be transported in such a leaf will therefore depend 
very largely on the average size of the transport areas in the leaf. 
The anatomical investigation showed, in the first place, that in the 
case of net-veined Dicotyledonous leaves, the conception which 
has been worked out above, completely explains the phenomena 
observed. Mr. Zijrsrra indeed found, that in these leaves veins, which 
take up the whole thickness of the leaf, are devoid of intercellular 
spaces. A leaf such as that of Dahlia, in which similar transverse 
veins occur only at comparatively long intervals, will have large 
transport areas, and will be able to form relatively wide strips of 
starch. On the other hand in leaves like those of Acer and Aesculus, 
in which numerous vein branches occur close together, and take up 
the whole thickness of the leaf, we can only expect to find narrow 
starch strips such as indeed occur. 
The transport areas, even in the Dahlia leaf, which isin this respect 
in the most favourable condition among reticulate leaves, are neverthe- 
less very small, certainly much smaller than 3 cm. in diameter, 
as simple inspection of the leaf shows. It is therefore evident, that 
in the first apparatus, in which the part of the leaf under the mercury 
measured 3 em, these leaves were bound to give negative results, 
as regards the conduction of carbon dioxide supplied to the leaf base. 
In all experiments with these leaves, an idea of the carbon dioxide 
transportation could, however, be obtained from the starch-formation 
which took place at the expense of respiratory carbon dioxide, 
derived from other parts of the same transport area. It is also 
evident that in these leaves the edges of the starch strips must often 
follow the irregular course of the veins and must suddenly cease at 
the veins. Only in those cases, in which only a small portion of the 
ieaf area was in the dark and so could produce but little carbon 
dioxide, it might be expected that the large lighted portion could not 
fill itself completely with starch, and that the edge of the starch 
strip would not be sharp. Places in which this could be observed, 
were indeed pretty frequent in Dahlia leaves. 
An important question now arose, as to the condition of the various 
parallel-veined leaves, which, in the experiments described above, 
aS = 
Proceedings Royal Acad. Amsterdam, Vol. XI. 
