1904] Soirees. 



21 1 



place, viz., the root-hairs, where the water enters ; the deeper 

 tissues, the fibro-vascular bundles of root and stem, through which 

 the water ascends ; the veins of the leaf, along which it proceeds 

 to the intercellular spaces, at the exit of which stand the guard 

 cells of the stomata. 



It was pointed out that sap is a dilute solution of food, partly 

 of a mineral nature, taken from the soil, partly of an organic 

 nature, derived trom carbonic acid absorbed by the leaves from 

 the atmosphere. The chief mineral constituents are phosphates, 

 silicates and nitrates of potassium and calcium. The chief organic 

 constituents are sugar, soluble proteids and organic acids, with 

 sometimes coloring matters. These are conveyed or carried about 

 in the water, which has the double function of dissolving and dis- 

 tributing this food material. An enormous amount of water takes 

 part in these two processes. Besides the above uses of the water, 

 it is itself food material, and also serves to render the plant turgid, 

 thereby enabling succulent plants to stand erect. The proportion 

 of water in plant tissues is very large, from 40% to 90%, but the 

 amount of water so represented is very small compared with that 

 which passes through the plant or tree and is lost by transporta- 

 tion. For every pound of dry matter stored up in the tissues 

 between 300 and 400 pounds of water pass out into the atmos- 

 phere by the stomata. 



The causes of the upward movement of sap were next dealt 

 with. The water enters the root-hair from the soil by osmosis. 

 This osmotic action may be experimentally illustrated by placing 

 a strong sugar solution in a glass cylinder (lamp glass), one end 

 of which is covered by a bladder, and suspending the cylinder in 

 water. To eliminate purely hydrostatic action which might pro- 

 duce movement, have the level of water inside the bladder the 

 same as that without. In a short time the level inside rises, 

 showing that, it an interchange is taking place, more water is 

 entering, than escaping from, the bladder. Some sugar makes 

 its way to the outside liquid. Thus, on purely physical grounds, 

 one may see how water enters the root hairs of plants. Having 

 entered, it filttrs through to the deeper cells, and, in the case of 

 the higher plants, ascends chiefly by means of the Xylem elements 

 of the fibro-vascular bandies. This ascent is aided by osmotic 



