53 
On the Assimilatory System. 
G. Haberlandt, in Ber. d. deutsch. bot. Ges., Vol. IV, pp. 206- 
236, pl. xi., furnishes a valuable contribution to the elucidation of 
this interesting subject. 
E. Stahl and other authors have advanced the theory that the 
peculiar shape and position of the palisade-cells are due to the 
influence of insolation. The chlorophyll grains in the layers of 
the leaf most exposed to the sunlight are said to have the tend- 
ency to avoid the intense light of the perpendicular rays by 
placing themselves along the side-walls of these cells. They will 
thus assume a profile position, while the granules in the more pro- 
tected, deeper layers, in the so-called spongy parenchyma, will 
occupy the walls parallel to the leaf-surface, thereby presenting 
their broad side to the incident light. This tendency, 
according to the authors cited, will cause the palisade-cells to 
elongate in a direction perpendicular to the leaf-surface, so as to 
afford the chlorophyll grains ample room to assume the most 
favorable position. The cells on the spongy parenchyma on the 
other hand will, for the same reason, expand horizontally. Thus 
the frequent absence of palisade tissue and the more abundant 
development of the spongy parenchyma in leaves that have 
grown in the shade would be explained. 
When the solar rays strike the surface of a leaf perpendicular- 
ly, the migration of the chlorophyll grains to the side-walls of 
the palisade-cells, can be easily observed zx some plants.* Ran- 
unculus Ficaria is said by Haberlandt to be very good for this 
purpose. The chlorophyll grains will leave the transverse walls 
completely in about fifteen minutes. It takes several hours, 
how ever, for them to return to their former position after insola- 
tion has ceased. Ifa cross-section from a leaf that had been inso- 
lated for some time is examined, the chlorophyll grains are found 
distributed rather uniformly on all the side-walls of the palisade- 
cells. Then, if the sunlight is directed so as to fall perpendicular- 
ly on the long Sides of these cells, nearly all the chlorophyll grains 
will gather on those side-walls which lie in the same plane as the 
*Prothallium of ferns, Vaucheria (Stahl), Ornithoga/um nutaus and umbellatum, 
Muscari racemosum, Scilla bifolia, etc. 
