THE ABSORPTION OF WATER 



29 



figure shows the surface cells {ee) of a root, from one of which a single root- 

 hair (M) has grown out. ' The bodies deeply shaded are microscopically minute 

 particles of soil between which are shown airspaces left white. Each soil 

 particle is surrounded by a thin layer of water held fast by surface attraction ; 

 where the attraction of neighbouring particles of earth co-operate at their 

 re-entering angles these, otherwise thin, layers of water form thicker accumu- 

 lations. These aqueous spheres are indicated in the drawing by wavy lines. 

 The surface of the roothair is also (at a) covered by a thin layer of water, and 

 its walls are saturated with it. Let us now regard the roothair for a moment 

 as inactive, and assume that no disturbance at all is taking place in the soil ; 

 then all the aqueous spheres of the soil particles are not only in contact with 

 each other, but are also in equilibrium.' 



* If we now assume that the roothair, hh, absorbs water at a, its surface layer 

 in that situation will have less water than corresponds to its power of attraction; 

 it withdraws water first of all from its immediate neighbourhood and, in conse- 

 quence, the equilibrium in these situations will be disturbed. This disturbance 

 spreads outwards on all sides until 

 the molecular equilibrium of all the 

 aqueous spheres is re-established. 

 By this means they all become 

 thinner and thinner and the soil as 

 a whole drier. This desiccation, 

 however, may make itself evident 

 not merely in the immediate neigh- 

 bourhood of the roothair, but will 

 at the same timeaffect more distant 

 parts. Every roothair becomes 

 thus a centre of a current directed 

 towards it from all sides, and at 

 the surface of a small root covered 

 by thousands of roothairs a similar 

 movement results which directs 

 the aqueous particles in the soil 

 from all sides, but more especially 

 radially, towards the axis of the 

 root.' The root is thus capable of 

 making use of layers of soil although it may not actually be imbedded in 

 them. ' If we assume the aqueous envelope of a particle of soil to consist of 

 several very thin layers, then the aqueous molecules lying nearest to the 

 particle of soil will be attracted with maximum force, and this attrac- 

 tion becomes progressively less in the successive external layers until, in the 

 outermost layer, when the soil is saturated with water, the molecular attraction 

 is only just great enough to prevent the water from trickling away. When the 

 water disappears at a or at y8, y, &c., the outermost layer of the aqueous spheres, 

 more especially, moves first, because it is the one least firmly held and most 

 easily put in motion. The more water the roothair has already taken up the 

 thinner are the aqueous spheres of the entire system, and the greater is the force 

 with which the primary layer — now outside — is held ; so much the greater 

 must the force be which can pull the water into the wall of the roothair, 

 and the more difficult and slower the transmission of a disturbance from a to 

 /?, y, 8. A condition of the aqueous envelopes may finally ensue where all the 

 primary layers are held so firmly by the soil particles that no more water 

 can enter the wall of the roothair.' 



When this degree of drought in the soil is reached then the aerial parts of 

 the plant must obviously wither though transpiration be prevented as much as 

 possible. Sachs found withering took place in tobacco plants grown in different 



Fig. 5. Roothair, /z/«, in the soil. Diagrammatic. For expla- 

 nation see text (simplified from Sachs's Experimental Physio- 

 logy). In the figure the thickness of the adhesion layers is 

 greatly exaggerated ; they cannot be distinguished micro- 

 scopically. 



