Permeability 147 
for if the crystalloids only diffuse slowly into the protoplasm, while 
the membrane offers very little opposition to the passage of water, 
absorption of water will take place readily at first. 
In young plant cells vacuoles are either non-existent or very 
small, and the system thus approaches very closely to such a cell as 
that considered above, but surrounded by a wall. The behaviour of 
such a cell towards water is similar to that already described, but the 
presence of the wall is responsible for certain differences. Thus, when 
the walled cell is immersed in water or in a dilute solution, water is 
absorbed, but this cannot proceed as rapidly or to the same extent 
as in the case of the unwalled cell, for as water passes into the cell 
the increase in volume involves a stretching of the cell wall which 
thus exerts a continually increasing tension inwards, so that on 
account of this water tends to be forced out from the cell. When 
these two forces, that forcing water in and that forcing it out, equal 
one another, an equilibrium condition is reached, and the protoplast 
is thus prevented from absorbing water until it bursts. With thin 
cell walls this can however sometimes happen. Thus some marine 
algae, notably Derbesia and Bryopsis, when transferred to fresh water 
(Noll, 1888), will burst very easily. Lidforss (1896) has recorded the 
bursting of pollen grains of a number of species, notably those be¬ 
longing to the Liliaceae, in the same way. F. E. Lloyd (1915, 1916, 
1917 a, 1917 b) has observed the bursting of pollen tubes of Lupinus, 
Lathyrus and Phaseolus in distilled water or weak sucrose solutions. 
The same author has even observed the bursting of pollen grains of 
Gossypium in 50 per cent, glycerol, 25 per cent, sucrose and 0*45 N 
potassium nitrate, surprisingly high concentrations. The tips of the 
hyphae of moulds (Curtis, 1900) and the asci in some Ascomycetes 
(Livingston, 1903) will burst if placed in distilled water or in a very 
weak solution. 
The water relations of the non-vacuolated cell can thus be easily 
explained if it is assumed that the outer layer of the protoplast 
consists of a membrane which is readily permeable to water but 
impermeable, or only comparatively slightly permeable, to many 
soluble substances present in the protoplasm. Such a view, as has 
already been stated, has its opponents, one of the most ingenious of 
whom, M. H. Fischer, has attempted to explain the absorption of 
water by organic tissue on account of the colloidal nature of the 
tissue. In a long series of experiments Fischer (1910) has shown that 
the swelling of certain proteins, namely gelatine and blood fibrin, in 
water and in solutions of acids, salts and alkalies, closely resembles 
10 -2 
