IN VEGETABLE BIOLOGY. 879 
quite free of chlorophyll. In sunlight the grains first move on to 
the side-walls and then into the arms, where they ultimately 
mass upon or near the septa. Now in sunlight assimilation is 
more rapid than in diffused light; but on Haberlandt's theory 
precisely the opposite of what occurs would have been predicted, 
viz. that increase in the rapidity of assimilation would be accom- 
panied by greater facilities for—or at the very least not signalized 
by the interposition of obstacles to—the passage of its products 
from cell to cell. Again, it is a remarkable fact that when cells 
stretch in the plane of the leaf and not perpendicularly thereto, 
as is the case with many aquatics, the chlorophyll is disposed, in 
diffused light, upon the upper and lower walls, leaving the side- 
partitions free from it. Under these circumstances the assimi- 
lated matters of, say, an Elodea-cell are free to pass either out- 
wards towards the edge or inwards towards the midrib, or apically 
towards the tip, or basally towards the bottom of the leaf. In 
good diffused light and in the sun’s rays the grains are apostro- 
phized and rotation sets in, the four walls previously bare being 
now traversed by the protoplasmic stream carrying with it the 
chlorophyll, and the two previously chlorophyll-studded walls 
being now bare. If Haberlandt’s theory be correct, the products 
of assimilation are now free to move only upwards or downwards 
(i. e. outwards or inwards), and not laterally towards the mid- 
rib: consequently difficulties so great as to put a positive em- 
bargo on transport accompany an increased rate of assimilation. 
Further, Haberlandt himself cites instances of palisade-tissues, 
the arrangement of the chlorophyll of which is such as, upon his 
theory, to point to lateral transport as the advantage derivable 
therefrom. But direct conveyance is, according to him, ensured 
by, firstly, radial disposition of the assimilating tissues with 
respect to the vascular bundles; and secondly, absence of chlo- 
rophyll from a septum or septa nearest to the vascular bundle. 
One of these cases of presumed lateral transport has just been 
noticed; another, figured by Haberlandt*, shows the transverse 
septa between straight palisade-cells deprived of chlorophyll. I 
venture to give the following reason for this disposition. It has 
been shown that the chlorophyll of palisade-cells tends to mass 
in darkness and in the sun’s rays; and seeing that photolytic 
movements are induced in the mesophyll-cells of the higher aero- 
phytes at low grades of illumination, it is most probable that the 
.* See fig. 8 of his 1886 memoir (Cirsium pannonicum). 
