194 
Walter Stiles 
in leaf cells and attributes this, no doubt correctly, to the formation 
of carbohydrates. Meier (1915) found in a number of species a some¬ 
what higher osmotic pressure in the leaves exposed to the sun than 
in leaves of the same species growing in shady habitats. Observations 
by Ursprung and Blum (1916 c) on Funaria showed the same relation, 
while the same authors found higher osmotic pressures in epidermal 
cells and most of the mesophyll cells of sun leaves of the beech as 
compared with the corresponding values found in shade leaves of the 
same species. Intense illumination thus appears to lead to an increase 
in the osmotic pressure of leaf cells, as is indeed generally supposed. 
Periodicity in the Values of Osmotic Pressure in Plants 
The various factors of the environment undoubtedly influencing 
the osmotic pressure of plant cells, it is only to be expected that in a 
climate with a daily and annual periodicity there should be a corre¬ 
sponding periodicity in the value of the osmotic pressure of the cell 
liquids. A daily periodicity in the value of the osmotic pressure of 
the cells of the different tissues and in different parts of the plant 
has been shown by Ursprung and Blum (1916 b) in Helleborus fcetidus, 
Fagus, sylvatica and Sedum acre and also in the leaves of Funaria, the 
osmotic pressure increasing from early morning until the afternoon 
and then falling until early the next morning. This course runs 
parallel with the temperature and inversely as the humidity. 
An annual periodicity has also been made out by the same 
authors, their results indicating on the whole minimum values in 
the summer and maximum values in the winter. There are, however, 
many departures from this, as would be expected when one considers 
that the changes in the various environmental factors are by no 
means regular. 
The Substances in the Plant responsible for the 
Osmotic Pressure 
It has been noted in Chapter VI that an osmotic pressure can 
only be developed when two solutions are separated by a membrane 
which is impermeable, or only little permeable, to solute or solutes 
on one side of the membrane. On the simple osmotic view of the 
plant cell we have therefore to suppose that in the vacuole there are 
present solutes to which the protoplasm is impermeable, and that 
these solutes are in such concentration as to give rise to the high 
osmotic pressures noted in previous sections of this chapter. To de¬ 
termine the substances responsible for the osmotic pressure de Vries 
