Permeability 55 
researches of W. Gardiner (1884), A. Meyer (1896), Strasburger 
(1901) and A. W. Hill (1901) it is clear that fine cytoplasmic threads 
penetrate the pit membranes, and sometimes indeed the whole 
thickness of the cell wall, so that in spite of the presence of the cell 
wall there is actually a continuation of protoplasm throughout the 
plant. Cellulose walls readily absorb water, and in the living plant 
the cell wall is normally permeated with imbibed water. Cutinised 
and suberised walls are however more or less impermeable to water, 
and their principal function is to prevent loss of water from the 
surface of cells. 
Such then is the system with which we are concerned in a con¬ 
sideration of the problems of permeability. This system is very 
varied, attaining its highest complexity in the adult plant cell. In 
the latter we have to recognise at least three phases, the cell wall, 
the protoplasm and the vacuole. Each of these moreover is itself 
a complex system, both the cell wall and protoplasm each con¬ 
taining a more watery phase and at least one other phase, while 
there is evidence that the cell sap in the vacuole may also contain 
a colloidal disperse phase as well as water with substances in pure 
solution. At the boundaries between outer medium and cell wall, cell 
wall and protoplasm, protoplasm and vacuole, there are separating 
layers which there is every reason to believe have different pro¬ 
perties from the bulk of the phases they separate. Further, in both 
the cell wall and protoplasm, and perhaps also in the vacuole, we 
have at least two-phase and probably polyphase systems in which 
there are consequently relatively large surfaces of contact between 
the phases. The constituents of the different phases vary from plant 
to plant and from tissue to tissue in the same plant. 
We thus see how much phase boundaries figure in the structure 
of the cell, and it is impossible to lay too much stress on the im¬ 
portance that surface phenomena must play in regard to cell per¬ 
meability. Before passing on to problems of the cell itself it will 
therefore be necessary to discuss briefly the more important facts 
relating to surfaces, as well as other physical and physico-chemical 
principles with which acquaintance is necessary for a proper realisa¬ 
tion of permeability phenomena. 
(To be continued.) 
2—6 
