68 VEGETABLE PHYSIOLOGY 
The original state of equilibrium has been disturbed by the 
shock to the protoplasm administered by the stimulation, 
and the latter allows or compels the water to pass outwards. 
The active influence of the protoplasm is seen also in 
another class of phenomena. Certain structures known as 
nectaries occur conspicuously in many flowers. They are 
aggregations of cells of a particular kind which exude a 
sugary fluid upon their surface. The liquid in the cells 
contains a little sugar, and this weak solution is capable 
of passing through the protoplasm, not by osmosis, but by 
a kind of filtration. Its concentration is usually increased 
by subsequent evaporation of the water in which it is dis- 
solved, so that the secretion when collected has a distinctly 
sweet taste. When the petals of certain flowers bearing 
these nectaries are cut off, and their cut ends immersed in 
water, the glands continue for some time to exude the 
nectar. There can be no question here of a gross filtration 
of water under pressure through the tissue, as there is no 
such pressure acting on the base of the cut petal. The 
protoplasm causes a stream of water to flow into the cells 
of the gland by producing osmotic substances inside them, 
in this cage chiefly sugar. The turgescence thus set up in 
the gland cells exerts a strong hydrostatic pressure on the 
limiting membranes of these secreting cells, which ultimately 
so stimulates the protoplasm as to cause it to allow the 
sugary solution to exude upon their free surfaces. We can 
discriminate between two forces at work in the excretion of 
the nectar. The absorption of water by the gland cells is 
due to osmosis ; the excretion from them on to the exterior 
of the gland is more a question of a modified filtration 
under pressure from the turgid cell. This is shown by the 
fact that if the surface of the gland is carefully dried, the 
exudation shortly recommences. Osmosis is not possible 
under these conditions. If the gland is killed by alcohol, 
the sugar already there is retained in the cells, and no 
exudation of nectar, or even of water, takes place. 
The vital activity of the protoplasm is thus seen to be 
