THE MOVEMENTS OF FLUIDS IN STEMS. 205 



II. The sap — what is it 1 



The use of the word sap is so entirely connected with the well- 

 known but exploded theory of the ascent of crude, and the descent 

 of elaborated sap, that it is as well to discard it at once as a 

 scientific term, although we may continue to use it in a popular 

 manner. It may be useful to give a popular definition of the 

 wox - d "sap," and it may perhaps be defined in the least objection- 

 able manner, by saying that it is " Water carrying constructive 

 materials in solution or susjiension." The movement of fluids in 

 plants is a problem of great complexity, hence the still lingering 

 longing after the simple theory of the ascent of crude and the 

 descent of elaborated sap. Scientifically the sap is known as the 

 cell-sap, that is, the water, with substances in solution and suspen- 

 sion which permeates and saturates the wall and protoplasm of 

 the cell, as well as accumulates in drops, or forms a large mass in 

 the vacuoles or central cell-sap cavity of the cell. When the cells 

 lose their contents and become filled with air, a portion of cell-sap 

 (in its widest sense) will remain in the wall, depending on the 

 chemical and physical nature of the cell- wall. In such a plant as 

 Elodea Canadensis, the common water-pest, the movements of 

 fluid ai*e comparatively simple. The plant takes in water from 

 the external medium, the water containing mineral matters and 

 gases in solution. These pass from cell to cell. Constructive 

 matters are elaborated in the cells containing chlorophyll. They 

 are modified in different ways, and carried to such parts of the 

 plant as are in a state of growth and require the so-called con- 

 structive matters, or those necessary for the formation of new 

 cell- walls and new protoplasm. The slow movements in such a 

 plant are entirely determined by (1.) the position of the source of 

 supply, and (2.) by the position of the pai-ts requiring the nourish- 

 ment. Thus a very young green leaf may supply matter for the 

 growing apex of the stem, while leaves still further from the apex 

 will supply matter for the growth of the youngest adventitious 

 root. In such a simple water-plant no viood exists in the fibro- 

 vascular bundles, the cells of the xylem retaining their cellulose 

 walls, the walls not becoming ligneous; hence the complication in 

 the movement of fluid introduced by the presence of wood is 

 thus avoided. In plants with a hard massive trunk, in which the 

 wood part of the fibro-vascular bundle is enormously developed, 

 we have a special rapid current of water in the xylem of the fibro- 

 vascular bundles alone, in addition to the slow movement of 



