362 HOW CROPS GE'OW. 



Rapid Motion of Sap in the Stem, In the stem of the 

 plant we have commonly a resistance to root-action", so far 

 as a flow of liquid is concerned. The ducts and sieve- 

 cells, in conifers, the wood-cells though offering visibly 

 continuous channels for the transmission of juices, are 

 nevertheless in most cases extremely small, and while they 

 raise liquids with enormous capillary force, they retain 

 them with the same force, and continuous motion can only 

 be the result of a correspondingly energetic disturbance. 

 The root-action winch can sustain a column of mercury 

 many inches, or one of water many feet high, in a wide 

 tube, is greatly neutralized by capillarity as we ascend the 

 stem from the root, or the root from its young extremities. 

 Root-action is, however, unsteady in its operation, and 

 when it declines from any cause, it is capillarity which acts 

 rapidly within the ducts and visible channels to supply 

 waste by evaporation. 



Motion of Nutritive or Dissolved Matters : Selective 

 Power of the Plant. The motion of the substances that 

 enter the plant from the soil in a state of solution and of 

 those organized within the plant is to a great degree sep- 

 arate from and independent of that which the water itself 

 takes. At the same time that water is passing upwards 

 through the plant to make good the waste by evaporation 

 from the foliage, sugar or other carbohydrate generated 

 in the leaves is diffusing against the water, and finding its 

 way down to the very root-tips. This diffusion takes place 

 mostly in the cell-tissue, and is undoubtedly greatly aided 

 by osmose, i. e., by the action of the membranes them- 

 selves. The very thickening of the cell-walls by the dep- 

 osition of cellulose would indicate an attraction for the 

 material from which cellulose is organized. The same 

 transfer goes on simultaneously in all directions, not only 

 into roots and stem, but into the new buds, into flowers 

 and fruit. We have considered the tendency to equaliza- 

 tion between two masses of liquid separated from eacl* 



