EELATION OF WATER TO THE PROTOPLASM 63 



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 nitration 

 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 the sugary 

 solution to exude upon their free surfaces. We can dis- 

 criminate 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 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 

 intimately connected with the presence of water in its 

 substance. The importance of the ready access of the 

 latter is seen further from other considerations. We have 

 incidentally alluded more than once to the fact that the 

 liquid concerned in these osmotic currents is not pure 

 water only, but should rather be regarded as an extremely 

 dilute solution of various salts. Though the protoplasm 

 opposes the passage of anything like a strong solution of 

 inorganic salts, it allows very dilute ones to enter the cell; 

 much as it does pure water. In this way the slowly dif- 



