48 LECTURE IV. 



brief account of the general nature and properties of the soil. 

 The soil may be regarded as consisting of a mixture of 

 irregular particles of mineral and of organic matter (humus). 

 The interspaces between the particles are usually filled 

 with air, but even when it is very dry, each particle is covered 

 with a film of water, hygroscopic water as Sachs terms it, which 

 adheres to it with considerable force. The root-hairs make 

 their way into these interspaces, and, in the course of their 

 growth, their cell-walls come into very close contact with the 

 particles of the soil. In consequence of this intimate con- 

 nexion, they can readily absorb the hygroscopic water of the 

 particles, although they have to overcome the force of ad- 

 hesion existing between the particles and the water which 

 invests them. Inasmuch as the particles with their films of 

 water form a sort of capillary system in the soil, the with- 

 drawal of water by a root-hair at any point causes a flow 

 of water towards that point from adjacent particles : in this 

 way a plant with a well-developed root-system drains a 

 considerable area. The roots, as they grow and branch, form 

 root-hairs at new points, so that fresh sources of supply are 

 continually being opened up. 



This property of retaining water is not possessed equally 

 by all kinds of soil: it is possessed, for instance, to a high 

 degree by clay and to a low one by sand. Moreover, the soil 

 can not only retain a portion of the water which enters and 

 passes through it in the liquid form, but it can condense 

 aqueous vapour. 



The importance of this latter property of the soil is well shewn by an 

 experiment of Sachs. A scarlet-runner was grown in a flower-pot, and, 

 after having been left unwatered for some time, the pot was placed in a 

 receiver, the air in which was saturated with aqueous vapour, the stem and 

 leaves projecting into the external air through an aperture in the lid of 

 the receiver. The plant, which was beginning to wither, soon recovered 

 a healthy appearance. From this it is evident that the plant must have 

 been supplied with water; and since no water in the liquid form was 

 poured on to the soil, and since, as Sachs had previously shewn, roots 

 cannot directly absorb aqueous vapour, it is evident that the water with 

 which the plant was supplied had been derived from the watery vapour 

 with which the atmosphere of the receiver was saturated, and had been 

 condensed on the surface of the particles of soil. 



