382 MR. 8. LE M. MOORE’S STUDIES 
The remarks which follow, embodying my own views on this 
subject, may possibly commend themselves to some. 
(1) The almost universal occurrence of palisade-cells would 
seem to point to the existence of some fundamental structural 
principle which made itself felt as a factor in Natural Selection 
at a very early period in the phylogenetic history of the higher 
vegetation. This principle can be none other than economy of 
growth. How can the assimilating portion of the leaf be in- 
creased? Manifestly in two ways, and in two alone, viz. by 
superficial enlargement or by an addition to the depth of the 
tissue. Inerease of surface entails either coincident increase of 
the vascular system ora less effectual means of transport for 
assimilation-products, both of which disadvantages are obviated 
by the arrangement of the cells perpendicularly to the surface. 
(2) The agency whereby this perpendicularity is effected is 
undoubtedly light. 
(3) A contingent advantage of a palisade system is the mini- 
mizing of transpiration. Moreover, the cells may possibly be to 
some extent protected from the injurious effects of strong sun- 
light, which is known to be capable of destroying the protoplasm 
and chlorophyll of the overlying epidermal cells. 
(4) The relation of the assimilating to the transporting tissues, 
as maintained by Haberlandt and Heinricher, is not proven. The 
latter, as was before mentioned, has objected to Pick’s heliotropie 
ideas by explaining variations from the perpendicular, in cases 
where the direction taken is not towards the vascular bundle, as 
caused by mechanical dislocation; and it seems probable that 
the pressures and tensions exerted by the stretching bundle- 
elements might lead to some such radial arrangement as that 
upon which Haberlandt and Heinricher rely. Moreover, the 
very large number of cases wherein no radial relation is manifest, 
forces one to adopt a purely mechanical as opposed to a nutri- 
mental theory of cell-arrangement in the leaf. 
It may be asked, if the direction of the streaming protoplasm 
be that of least resistance, how can the movement be regarded as 
phototactic? Does not the one view exclude the other? I 
venture to think not ; and for this reason. The capacity of light 
to modify the form of palisade-eells is admitted on all hands. 
This granted, what difficulty is there in conceiving that the form 
of all cells in direct relation with light is so ordained by this agency 
as to ensure, upon simple mechanical principles, the maximum ` 
