ATfACHMENT OF RüüT-HAIRS. 



^57 



a single root-hair, which has been protruded from a young root into the surrounding 

 soih 



In Fig. 205, e is tlie epidermis of a root which has grown perpendicularly down- 

 wards ; the root-hair h h has been developed as a protuberance of an epidermal cell, 

 and at 2 and s it is closely applied to single particles of soil, as seen below. The bodies 

 T, shaded dark, are microscopically small particles of earth, between which are cavities 

 containing air — left completely white. Each particle of soil is enveloped by a thin layer 

 of water, which is held fast by surface-attraction : where the attraction of neighbouring 

 particles of earth co-operates at the re-entering angles, these otherwise thin layers of 

 water form thicker masses. These aqueous spheres are indicated in the drawing by waved 

 lines (as at ß and y). The surface of the root-hair is also (as at a) clothed with a thin layer 

 of water, and its walls are saturated with that fluid ; the spaces left white are filled 



with air. Let us now regard the root-hair for a moment as inactive, and suppose no 

 disturbance at all to be taking place in the soil. Then all the aqueous spheres of the 

 particles of earth are not only in contact with one another, but also in equilibrium. 

 If we were to take away the layer of water at y, for example, the equilibrium would 

 be disturbed throughout the entire system, and water would flow from Ö and ß 

 and other places, towards y, until the forces were in equilibrium. If we now assume 

 the root-hair h h to absorb the water a or t, this penetrates through the membrane 

 into the interior of the hair, or it moves along t, a, S, in the substance of the wall 

 itself: the surface of this wall at a or r thus possesses less w^ater than corresponds to its 

 power of attraction. It attracts it, therefore, at the spot r : this then absorbs water from 

 ß, and the movement proceeds towards y and 8, Ac, until the molecular equilibrium of 

 all the aqueous spheres is again established. By this means they all become thinner 

 and thinner, and the soil as a whole drier. This desiccation, however, mav make itself 

 [3] 



