38 MISSOURI BOTANICAL GARDEN. 
property of semi-permeability (4). Moreover, such an ex- 
planation seems to accord better with facts than does the 
explanation based on the assumption that such a membrane 
exists. On the whole, the pressure developed in living cells 
does not obey the gas laws. In order to keep the osmotic 
theory of water absorption it is necessary to make various 
assumptions regarding the changes in the permeability of the 
assumed membrane. When we assume that a semi-per- 
meable membrane exists, and undertake to become familiar 
with its properties, we find that they are not known. The 
permeability of the same cell seems to be affected differently 
by different substances. It changes as the concentration of 
the medium changes and seems to vary, even with the season 
of the year. The semi-permeable living membrane is an 
assumption, upon which is based a theory, which, it seems to 
me, is not only no longer useful, but even detrimental to a 
correct understanding of the phenomena of absorption and 
secretion. As Martin H. Fisher (4) has pointed out, we 
know very little regarding the affinity between colloids and 
watery solutions. We use the word “affinity” to cover our 
ignorance, but it seems better to do this than to make an as- 
sumption that does not find justification in the facts that are 
known. 
I am greatly indebted to Dr. George T. Moore for the en- 
couragement and advice that he has given me while engaged 
in this work. 
BIBLIOGRAPHY. 
1. Cohnheim, Otto. Versuche iiber Resorption, Ver- 
dauung und Stoffwechsel von Echinodermen. Zeit. 
Physiol. Chem. 38:9. 1901. 
2. Czapek, Friedrich. Ueber eine Methode zur direkten 
Bestimmung der Oberflichenspannung der Plas- 
mahaut von Pflanzenzellen. Jena, Verlag von Gus- 
tave Fischer. 1911. 
3. Fernbach, M. A. Sur l’invertine ou sucrase de la 
levure. Annal. Institut Pasteur. 4: 641. 1890. 
