232 PART III. — VEGETABLE PHYSIOLOGY. 



sion. Moreover, the ascent of the sap is not mainly through long 

 tubes, but from cell to cell, from fiber to fiber, from duct to duct. 



It is evident that all movements of sap would soon cease on 

 account of the establishment of an equilibrium of fluids in the 

 cells, just as the osmosis of any two liquids through a membrane 

 ceases as soon as the liquids on either side of the membrane 

 cease to differ in composition, were there not in the growing 

 plant disturbing causes constantly coming into play. These 

 causes are various. 



One is the evaporation of water from the plant. This takes 

 place very largely from the leaves. It is greatly facilitated by 

 the enormous surface which these organs expose to the sunlight 

 and air, but it is also promoted by their structure. The numer- 

 ous stomata and the loose arrangement of the cells of the interior 

 of the leaf, permitting within it a free circulation of air, greatly 

 facilitate the process. The leaf-cells, thus losing a portion of 

 their water by evaporation, are able to take up more from the 

 cells below, and so the ascent of the sap continues. 



Another cause is the formation of new cells. This is constantly 

 taking place in various parts of the plant during the season of 

 growth, and, as the protoplasm of each young and growing cell 

 has a great avidity for water, it absorbs it greedily from adjacent 

 cells, and these from others, and so on. 



Every chemical change, also, and these changes are all the 

 time taking place in the living plant, more or less disturbs the 

 equilibrium of fluids, and so promotes their movement. If a new 

 soluble compound be formed, for instance in the leaf, it tends to 

 diffuse to all other portions of the plant, until equally distributed. 

 The change of substances of less density to those of greater 

 density, of crystallizable to colloid bodies, of liquids to solids, 

 or of solids to liquids, will each and all give rise to movements 

 of diffusion. It is probable, also, that the mechanical move- 

 ments of branches and leaves by the wind may be of consider- 

 able service in promoting the circulation of fluids in the plant. 



Gases in the Plant. The most important of the gases 

 found in the plant are carbon dioxide, oxygen, nitrogen and 

 watery vapor. These exist either in solution in the cell-sap, or 

 free in the intercellular spaces, and in the cavities of the vascular 

 tissues. 



