Permeability 203 
Inadequacy of the Simple Osmotic View of the Plant 
Cell as regards its Water Relations 
It will be observed that on the simple osmotic view of the plant 
cell it is assumed that the amount of water contained in the cell wall 
and the protoplasm undergoes no change with varying conditions, 
and that the only interchange of water takes place between the 
vacuole and the external medium. And even when the view is slightly 
modified so that the semi-permeable membrane separating the ex¬ 
ternal medium from the osmotically active cell sap is regarded as a 
thin limiting layer of the protoplasm instead of the whole of it, 
changes in the water content of the cell wall are neglected, while it 
is assumed that the protoplasm and the vacuole form one uniform 
solution as regards osmotic properties. It is also assumed that the 
swelling or shrinkage of the vacuole is solely to be accounted for on 
simple osmotic grounds. It is clear that we have no right to make 
these assumptions without good evidence, so that these questions 
deserve some little consideration. 
In this connexion the work of F. E. Lloyd cited in Chapter VIII 
is of particular interest as indicating the possibility of explaining the 
water relations of certain plant cells without the supposition of a 
surrounding plasma-membrane. Some recent observations of Lapique 
(1921) are also significant. This writer found that when filaments of 
Cladophora glomerata and C. oligoclona are immersed in acid solutions 
the cells become plasmolysed while the cellulose wall swells up and 
loses its sharp delimitation. The changes appear to be independent of 
the osmotic concentration of the external solution as they are ob¬ 
served with N/1000 hydrochloric acid as well as with N/10. Although 
alkalies appear to have no influence on the normal cell, yet they act 
on a cell treated with acid in the reverse sense to the acid. Lapique 
concludes from these observations that the acidity or alkalinity of 
the external medium is of more importance for the intake of water 
than the osmotic concentration of the medium. He further supposes 
that the cellulose wall and the protoplasm constitute colloidal sys¬ 
tems in which the particles carry different charges, as one is co¬ 
agulated by acids and the other by alkalies. 
Water Relations of the Cell Wall 
It is usually assumed that the cell wall is saturated with water and 
that this being so, the amount of water present in the cell wall is un¬ 
affected by conditions within very wide limits. It has, however, been 
shown earlier that the cell wall is not simple in chemical composition 
14—2 
