86 



as before we did so, which shows that the secretion of these 

 drops is perfectly independent of the rest of the plant and of 

 the root pressure, and depends on the local activity of paiticular 

 living cells. These cells form what are called nectaries, which 

 are well seen in the Crown Imperial (Fritillaria imperialis), 

 where they form large circular areas at the base of the perianth 

 leaves. They have no special pores, and generally lie more or 

 less on the surface, exposed freely to the action of the air, in 

 a sort of depression of the epidermis, which, however, does not 

 cover them ; thus a little cup is formed by cells, which take up 

 great quantities of water by the osmotic activity of substances 

 in their cell-sap from the end of the fibro-vascular bundle over 

 which their inner surface is always placed in direct contact, and 

 which breaks up beneath them in a radiating manner. The 

 activity of these cells is so great that more water is taken up 

 than the cells can contain ; they thus become turgid and a pres- 

 sure is set up in them, in consequence of which water is forced 

 out from them by filtration through the cell-walls, on to the 

 exterior at the surface of least resistance, in order to prevent 

 rupture taking place. In this case the cells appear to be able 

 of themselves to set up a pressure in the absence of the root 

 pressure which they were unable to do in the case of the water 

 gland. 



The causes of the ascending current of water are, then : — 

 I. — Internal causes, dependent on the plant itself: 



A. The endosmotic constant activity of the root- cells, 



producing a suction on the soil. 



B. Capillarity, which has a twofold action : i. Its action 



in raising a column of water. This will, of course, 

 have but little influence comparatively, except when 

 water and air occur together in the vessels, or in 

 the cases of root pressure in spring 2. Its action 

 in maintaining a column of water when already 

 raised. 



C. Diffusion in its widest sense. 



D. Imbibition. 



II. — External causes : — 



