290 Delf. — Studies of Protoplasmic Permeability by 
tissue at the extreme tip and the base. The leaf has. no stereome and is sup- 
ported by the turgor of the thin-walled cells, more or less aided by the 
framework of slender vascular bundles. The epidermis is cuticularized and 
has particles of wax embedded in it which prevent the leaf from being easily 
wetted externally, even when kept under water for many hours. Imme- 
diately beneath it are two layers of typical palisade cells, arranged with 
their long axes perpendicular to the length of the leaf and occupying more 
than half the thickness of the tissue. Beneath these and at right angles to 
them is a single layer (or more in the region of the bundles) of shortly 
rectangular cells (a, A, Figs. 3, 4), containing fewer chloroplasts than the 
palisade cells, and within these are two or three rows of much more 
elongated cells (b, b ; in Figs. 3, 4), the innermost of which are colourless 
Fig. 4. Longitudinal section of middle region of onion leaf, taken between the bundles ; drawn 
with Zeiss D.D. and 5-5 objective (Beck). 
and contain only a watery plasma with a few degenerated plastids. On the 
interior of these cells are the collapsed and broken remains of the paren- 
chymatous cells which once occupied the central cavity. 
The mean isosmotic equivalent of the cell-sap was determined by 
applying the tissue- tension method of de Vries to the tissues previously 
well soaked in water, and was found to vary from 0*20 to 0*23 grm. M. cane- 
sugar, according to the age of the leaf, the older leaves giving the higher 
values. By repeated microscopic measurement it was found that at these 
strengths the only cells which show any sign of plasmolysis were the 
vertically elongated cells internal to the palisade (A and B in Fig. 4). 
For this purpose a longitudinal section was cut from a turgid piece of 
leaf mounted in water, and an uninjured cell of the interior selected and 
measured by a micrometer scale and a high-power lens. The section 
was supported under the cover-glass by two strips of paper, and was 
irrigated with solution of any concentration by means of a narrow strip 
of linen dipping into a beaker and resting on the stage of the microscope. 
