68 VEGETABLE PHYSIOLOGY 



The original state of equilibrium has been disturbed by the 

 shook 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 nitration. 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 case 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 



