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of the water relations of the plant cell. Of course, where the volume 
of the cell wall is very small in comparison with that of the vacuole 
the differences in the absorption of water by the cell wall under 
different conditions may be negligible; in other cases the cell wall 
must obviously be taken into account. 
Water Relations of the Protoplasm 
Just as the cell wall is capable of absorbing water by imbibition, 
so is the protoplasm. Indeed, Pfeffer (1900) states that “the cell- 
wall, protoplasm, and indeed all organized structures have the power 
of imbibing water and swelling.” Experiments with non-living sys¬ 
tems such as gelatine, agar-agar, proteins and mixtures of these and 
other substances, strongly suggest that the capacity of the proto¬ 
plasm for absorbing water is highly dependent on the concentration 
and nature of the solution with which it is in contact. Acids and 
alkalies appear to exert the greatest influence on the power of such 
systems to absorb water, both increasing the swelling capacity con¬ 
siderably. Very comparable results have been obtained by Loeb 
(1897, 1898), Fischer (1910) and Miss D. J. Lloyd (1916) with animal 
tissues such as frog muscle and sheep’s eyes. Whether such pheno¬ 
mena are capable of explaining completely the water relations of 
the non-vacuolate cell without the invocation of a limiting semi- 
permeable membrane to the protoplasm is a question which has 
been discussed earlier without our being able to come to any definite 
conclusion. Further evidence from experiments with plants is badly 
needed. But that protoplasm has such powers of swelling there can 
be no doubt, nor can it be doubted that the degree of swelling will 
be influenced by the solution exterior to it. Substances may also be 
present in the vacuole to which similar considerations apply. Where 
the volume of the protoplasm is very small in comparison with that 
of the vacuole, and where the swelling capacity also is not great, 
such swelling of the protoplasm can be neglected in comparison 
with the water exchange due to the simple osmotic relations of the 
cell. In other cases the swelling of the protoplasm may be an im¬ 
portant factor in determining the water relations of the plant cell. 
This appears to be the case, for instance, in many succulent plants 
such as the cacti, in which the presence in the cell of pentosan muci¬ 
lages may render the swelling of the cell contents the determining 
factor in the passage of water between the cell and the exterior. 
Similar swelling occurs in red algae (McGee, 1918). 
