Permeability 55 
solution, and that as the diameter of the pit is small the rate of diffu¬ 
sion through it is proportional to the diameter and not to the area 
of the pit, so that with decreasing area of the pit, the rate of diffusion 
decreases much more slowly. Similarly, the law for diffusion through 
a multi-perforate septum may also be presumed to hold. 
The presence of chitin in the cell walls of Fungi has already been 
noted. For a summary of the literature dealing with the composition 
of the cell walls of these and other lower plants, the walls of which 
exhibit many differences from those of the higher plants, reference 
should be made to the first volume of Czapek’s Biochemie der Pflanzen 
(1913), and for a description of the cell wall of some red algae a paper 
by Sauvageau (1920) may be consulted. 
Semi-permeable Cell Walls 
Besides cell walls which allow the passage of water and most dis¬ 
solved substances, and cutinised and suberised walls which are practi¬ 
cally impermeable to water, there exist cell walls which are readily 
permeable to water, but impermeable, or almost so, to such simple 
solutes as sodium chloride (Gola, 1905). Such membranes were noted 
by A. J. Brown (1907) in the grain of barley. Barley grains readily 
absorb water from aqueous solutions of sulphuric acid, hydrochloric 
acid, cupric sulphate, ferrous sulphate, potassium chromate, silver 
nitrate and potassium ferrocyanide, while chemical analysis of the 
solutions shows that the dissolved substance is not absorbed. It thus 
appears that there is a semi-permeable membrane enclosing the seed 
of barley. That this membrane is not a living protoplasmic one 
becomes clear from the fact that grains can exhibit the semi-per¬ 
meable action in strong solutions of such poisons as cupric sulphate 
and silver nitrate, which would inevitably destroy the protoplasm 
and its semi-permeable properties if they came into contact with it, 
while conclusive proof is obtained from the fact that grains killed 
by boiling still exhibit the property. 
It is a curious fact that iodine, in solution in potassium iodide, 
is able to penetrate the membrane without apparently injuring it, 
for the subsequent passage of sodium thiosulphate is still prevented. 
Nitric acid, on the other hand, appears to destroy the membrane, for 
this acid is excluded for a time, but subsequently enters the grain. 
Further evidence of the presence of a semi-permeable membrane 
in the grain of barley is given by the fact that the blue pigment con¬ 
tained in the aleurone cells of the grain of Hordeum vulgare, var. 
ccerulescens, undergoes no change in colour when steeped in a solu- 
