54 
incident rays, 7. ¢., they will assume the profile position; a few 
grains will-pass to the transverse walls. 
If in all plants, or, at least, in very many, the chlorophyll 
grains were affected by the solar light in this manner, Stahl’s 
theory would appear highly satisfactory, but the plants mentioned 
are rare exceptions. Generally the chlorophyll grains do not 
change their position, but occupy the side-walls of the palisade- 
cells without being influenced by the intensity or direction of the 
sunlight. Even the assumption that the chlorophyll grains of 
the palisade-cells always present their profile to the incident rays 
is not correct. There are very many plants the leaves of which 
have large air-spaces under their stomata. These spaces are 
bordered by palisade-cells that have their lower side-walls curved, 
frequently even horizontal. Now these horizontal portions of 
the side-walls are found covered with chlorophyll grains during 
the most intense insolation, when the granules must necessarily 
present their broad side to the incident rays. Scz//a bifolia fur- 
nishes a very good illustration.* 
_ Frequently some palisade-cells will be found projecting with 
their free, upper end into the air-spaces mentioned. In such 
cells the uppermost transverse wall is often densely covered 
with chlorophyll grains, although it is parallel to the surface of 
the leaf. Besides, it must be considered that the rays when pass- 
ing through the epidermis are doubtless much refracted and dis- 
persed, so that we cannot positively say at what angle they fall 
upon the chlorophyll grains. We know that in the open air, sun 
rays never rest on a perpendicular leaf for any considerable time. 
[f, therefore, for all these reasons, the profile position of the 
chlorophyll grains in relation to the incident rays cannot be. con- 
sidered as the normal one, it follows that the peculiar expansion 
of the palisade-cells at right angles with the leaf-surface must not 
necessarily have been developed simply to afford the chlorophyll 
grains greater facility in assuming that position. 
The author proceeds to show that no chlorophyll grains are 
found on the walls separating the palisade-cells from the cells of 
the next lower layer, and that, in this respect, it makes no differ- 
ence whether these cross-walls are vertical, oblique, or horizontal. 
*Texamined Urginea Scillz (S. maritima) with the same result. 
