= tions. 
56 
grains place themselves, with decided preference, along the walls 
bordering on the intercellular air-spaces* with which the tissues 
of the leaf are freely interspersed. (In this respect the leaf of 
Scilla is especially instructive; a transverse section parallel to 
the nerves shows that numerous air-passages are found not only 
underneath the stomata, but between the contiguous vertical 
walls of the palisade cells. Even such leaves which, at first sight, 
-seem to have no interstices between their palisade-cells, e. g. 
Pilocarpus, Actostaphylos and Senna, will be found to contain very 
narrow, but numerous air-passages along their vertical walls, es- 
pecially if sections parallel to the leaf-surface be examined). 
At the same time each cell of the assimilatory layers feels 
compelled, as it were, to struggle for its share in the sunlight, 
the source from which it receives all its energy. 
How can we imagine a plan better adapted to fulfil these 
two conditions, than that on which the palisade tissue is con- 
structed? Into a given volume of this tissue the greatest pos- 
_ sible number of cells is packed in such a manner that each pre- 
sents its upper surface to the incident rays, permitting them to 
pervade its whole interior, while, at the same time, it furnishes 
ample accomodation to the chlorophyll grains, on its long walls, 
where they have the best opportunity to come into contact with 
the carbonic acid in the air passages. Whether the sun rays 
strike the profile or the face of the chlorophyll grains is indiffer- 
ent (as shown by Haberlandt), but whether some grains lie in 
the path of the sap-current or not, seems to be immaterial also. 
We shall, ofcourse, most likely not find many chlorophyll 
grains along the crdss-partitions, because there are no air pas- 
sages adjoining those walls. 
That in leaves growing in the shade the palisade-tissue is not 
developed at all, or not so freely as in leaves exposed to direct 
sunlight, might simply be explained by the consideration that the 
force of one of the factors, producing the peculiar growth of the 
palisade-cells, is considerably diminished. In all organisms the 
development of such parts is retarded or checked, which, for some 
reason or other, are prevented from fully performing their func- 
Jos. SCHRENK. 
* Frank in Pringsh. Jahrb, f, Bot., Vol, VIIL., p. 299. 
