Primula vulgaris var. caulescens 239 
pollination in the garden. Those plants he has examined had all 
developed some giant pollen-grains—one of the recognised indications 
of hybridity. 
In regard to the flower of this back-cross hybrid: Prof. Heslop 
Harrison says that it presents points of difference from that of the 
caulescent variety of the primrose, but that, having never had 
flowers of the two plants in hand together for purposes of comparison, 
he is unable to state exactly in what respects they differ. His 
impression is, however, that, in the caulescent variety, the individual 
flowers are slimmer than those of the back-cross hybrid and are 
borne on longer stalks. 
But the fact that this back-cross hybrid (which probably does 
not occur everywhere) undoubtedly exists in no way alters the 
status of the caulescent variety of the primrose dealt with above, 
though it certainly adds to the difficulty in distinguishing it. 
PERMEABILITY 
By WALTER STILES 
CHAPTER XIV (< continued ) 
THEORIES OF CELL PERMEABILITY (, continued ) 
The Viscosity Theory 
It has been suggested by Spaeth (1916) that the permeability of 
the plasma-membrane is determined by its viscosity, which is itself 
determined by the degree of dispersion of the colloids contained in 
it. On this view increased permeability is to be regarded as due to 
increase in rate of diffusion consequent on decrease in viscosity (cf. 
Chapter iv). 
This theory must be regarded as a solution theory of permeability, 
as it regards permeability as determined by diffusion through the 
plasma-membrane; it does not, however, define the chemical com¬ 
position of the plasma-membrane and so might be applicable to any 
solution theory of permeability. 
A consideration of the state of affairs postulated by the viscosity 
theory shows how the ultrafiltration and solution theories can be 
