182 PART III. PHYSIOLOGY. [. 44 



in solution from without, there must be a distribution of the ab- 

 sorbed substances from the absorbent surfaces to the other parts. 

 Further, when the plant-body is differentiated into parts which do, 

 and others which do not, contain chlorophyll, there must be a 

 distribution of the produced organic substance from the former to 

 the latter. In plants of relatively low organisation, the distribu- 

 tion takes place entirely by diffusion through the cell-walls, 

 that is by osmosis, when the plant is multioellular : and even in 

 the highest plants diffusion plays an important part. 



With regard to the distribution of water and substances 

 absorbed in solution from without in the more highly organised 

 plants there is a special conducting tissue, the wood or xylem of 

 the vascular bundles, extending from the roots, the absorbent 

 organs, to the leaves, the transpiring organs (see p. 164.) 



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. 180). 

 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 

 xylem-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 root 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, 

 3,025 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 atmos- 

 phere, 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 xylem. 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 



