CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 701 



The Root -Pressure. The existence of the root-pressure can be 

 easily ascertained. It is manifested spontaneously by that exuda- 

 tion of drops on the margin of the leaves of low-growing plants 

 during the night, to which allusion has already been made (p. 699). 

 An artificial manifestation of it is induced in stems which are cut 

 across at a time when, owing to active absorption and feeble 

 transpiration, the plants are rich in water ; drops exude from the 

 xylera-vessels at the cut surface of that part of a stem which is 

 still in connexion with the root. A familiar case of this is the 

 " bleeding " of certain shrubs and trees when pruned in the 

 spring. It is possible, in this way, to estimate both the activity 

 and the force of the root-pressure. By collecting the water which 

 exudes from the cut surface of the stem, the amount of water 

 absorbed by the i-oot in a given time is determined ; and by 

 attaching a mercurial manometer to the cut surface of the stem 

 the force of the root-pressure can be measured. For instance, 

 8,025 cubic millimetres of liquid were collected from a Stinging 

 Nettle in 99 hours ; and the root-pressure required a column of 

 mercury 354 millimetres in height to counterbalance it : in other 

 words, the root-pressure of the Nettle was nearly half an 

 atmosphere, and was capable of supporting a column of water 

 about 15 feet high. 



The essential point in the mechanism of the root-pressure is 

 the forcing of liquid by filtration under pressure from the paren- 

 chymatous cells into the xyleni. The process is probably to 

 be explained somewhat in this way. When a^ certain degree of 

 turgidity is attained in the parenchymatous cells abutting on the 

 xylem, their motile protoplasm undergoes a molecular change, in 

 consequence of which it becomes permeable and ceases to offer 

 resistance to the escape of the cell-sap ; consequently, under the 

 elastic contraction of the distended cell-walls, a portion of the cell- 

 sap is forced out of the cell. This molecular change in the state of 

 aggregation of the protoplasm of the parenchymatous cells probably 

 takes place at more or less regular intervals, so that there is a 

 sort of rhythmic pumping of liquid into the xylem of the root. 

 From this point of view, the root-pressure of a plant is simply the 

 expression of the force of the elastic conti'action of the cell-walls 

 of the parenchymatous cells abutting on the xylem-bundles in the 

 root. 



With regard to the external conditions which affect the root- 

 pressure, the most important is the temperature of the soil; a 



