512 BOTANICAL GAZETTE [DECEMBER 
included sodium, calcium, and aluminium chlorides, Bordeaux red, 
eosin, and sodium hydroxide. 
This extraordinary impermeability is confined to the exterior 
cell walls of the epidermis, as will be seen by the following simple | 
experiment. A sheet of epidermis stripped from the scale and 
mounted in water on an ordinary microscope slide, then irrigated 
with a o.4 M sodium chloride solution, was strongly plasmolyzed 
within 30 seconds. In order that plasmolysis should occur, it was 
necessary that the plasmolyzing solute should pass into the space 
between the cell wall and the retracted protoplast. Some part 
of the cell wall is therefore freely permeable to sodium chloride. 
Pieces of the scale, about 2 cm. square, with the epidermis still 
in place, were then placed in a 0.4 M sodium chloride solution. 
At intervals up to 30 minutes pieces were withdrawn, the surface 
dried with filter paper, and a small piece of epidermis from near 
the center of the piece of scale removed. These were placed 
between a microscope slide and cover slip, no water being added, 
and in all cases their cells were found to be wholly normal in appear- 
ance; but a few seconds’ irrigation with an o.4 M. sodium chloride 
solution now sufficed to cause violent plasmolysis. These experi- 
ments show that the exterior walls of the epidermal cells form a 
continuous layer highly impermeable to most substances and com- 
parable to certain seed coats as described by previous investigators.’ 
Summary 
1. The exterior cell wall of the epidermis from the inner surface 
of onion bulb scales is slightly permeable to hydrochloric acid, 
while it is practically impermeable to various salts, dyes, and to 
sodium hydroxide. 
2. It is necessary to consider the influence of impermeable cell 
walls in interpreting experiments on the permeability of plant 
tissues. 
LABORATORY OF PLANT PHYSIOLOGY 
HARVARD UNIVERSITY 
‘Cf. Brown, A., The selective permeability of the coverings of the seeds of 
Hordeum vulgare. Proc. Roy. Soc. London, B 81:82. 1909; SCHROEDER, H., 
die selektive permeable Hiille des Weizenkornes. Flora 102:186. rgtt; SHULL, C. A., 
Semipermeability of seed coats. Bot. GAz. 56:169-199. 1913. 
