55 
As the current carrying along the products of assimilation,—the 
carbohydrates—passes through these walls on its way to the con- 
ductive parenchyma surrounding the fibro-vascular bundles, it 
appears, so the author reasons, that the chlorophyll grains 
avoid those walls, so as not to obstruct that current. Conse- 
quently the other walls, z. ¢., the side-walls, must furnish space 
for the chlorophyll grains, hence their enlargement in the di- 
rection of the current. 
In the third and fourth sections of the article the author adds 
a considerable amount of material against Stahl’s (and Pick’s) 
theory. The most important fact he adduces seems to be, that 
palisade tissue is formed on both sides of leaves growing horizon- 
tally, and that some leaves, although they have grown in deep 
‘shade, will produce such tissue even on their lower side. This 
‘certainly seems to tell against the theory that the development 
of palisade tissue is dependent on the intensity of insolation. 
The last section is devoted to the discussion of the author's 
own theory, according to which the assimilatory tissue is con- 
structed on a plan that permits the transportation of the products 
of assimilation the most direct way and in the shortest possible 
time. The radial arrangement of the chlorophyll. carrying cells 
in Cyperus, several grasses, and Lguzsetum is illustrated and ex- 
plained, in order to demonstrate that the direction of the current 
_ within the cells, not their position relative to the surface of the 
organ to which they belong, determines their shape and size. 
* * * 
When I read Haberlandt’s paper and wrote an abstract of it 
for the BULLETIN, the theory advanced in it seemed to me very 
plausible indeed. But after having carefully examined the leaf 
of Urginea Scilla, both during insolation and after long con- 
tinued exclusion of light, I find that in many palisade-cells 
chlorophyll grains are found on the cross-partitions. In fact, I 
cannot see how the presence of a few granules at those walls 
could materially obstruct the passage of the sap-current. 
I venture to suggest that neither Stahl’s nor Haberlandt’s 
theories are necessary to explain the structure of the palisade- , 
cells. It is agreed, I think, that an indispensable factor in the — 
process of assimilation is the atmospheric air. The chlorophyll 
