IMPORTANCE AND PROPERTIES OF THE SOIL 165 



(anaerobic bacteria, &c.), which then develop in abundance, may directly or 

 indirectly injure the root-system or retard its development. 



In a normal soil, permeated throughout by moisture, the water is 

 held by capillarity, just as it would be in a system of capillaries containing 

 alternating columns of air and water. A very thin film of water is retained 

 with great tenacity by the molecular forces exerted on the wetted surface 

 of each soil-particle, and the same is also the case with regard to the 

 water absorbed by any organized bodies capable of swelling, which may 

 be present in the soil. The elongating roots and root-hairs push them- 

 selves between the soil-particles, as do also the rhizoids arid nutritive 

 hyphae of cryptogamic plants, and indeed all growing parts which are 

 buried in the soil. Each growing apex follows the path of least resistance, 

 and hence its course is often extremely sinuous, but when once penetration 

 is assured, roots may exert considerable lateral pressure as they increase 

 in thickness. In Fig. 16, it may be seen that the root-hair comes in contact 

 with air (a) at one part, and with water at another, but even at (a) the 

 cell-wall will remain fully permeated with water, since the air spaces are in 

 general saturated with water-vapour, and since a thin film of water will 

 be present as a general rule on the outer surface of the root-hair, thus 

 preventing direct contact between it and the surrounding air. A similar 

 film is also interposed wherever the root-hairs touch soil-particles (s, /), 

 and hence even at those points an absorption of water and dissolved 

 substances into the interior of the cell is still possible. This is also the 

 case when, as represented in Fig. u (p. 151), the root-hairs or rhizoids 

 have grown around and partially enclosed soil-particles, causing the 

 root-hairs to assume remarkable shapes, and rendering a separation of 

 the particles impossible without injuring the latter. 



The water absorbed by the root is replaced by fresh water from the 

 surrounding zones, and these again draw upon regions further removed, 

 so that in- this way, as was first shown by Schulz-Fleeth J , plants may 

 draw upon tracts of soil into which no portion of the root-system actually 

 penetrates. As the percentage of water present in the soil decreases, the 

 plant absorbs its necessary supply with greater and greater difficulty, 

 not only because the water continually decreases, but also because the 

 particles of fluid are more and more firmly held, as the soil dries. Hence 

 a condition is ultimately reached, in which the plant is unable to withdraw 

 from the soil the moisture still present in it ; and just as the plant is 

 unable to obtain by condensation from a saturated atmosphere sufficient 

 water for all its requirements, so also is a dry soil unable by the 

 condensation of water-vapour to become sufficiently moist to supply the 

 plant with fluid water. It is, however, easy to understand that a plant 



1 Schulz-Fleeth, Der rationelle Ackerbau, 1856, pp. 131 and 168. 



