240 A TEXTBOOK OF BOTANY [Cn. V, 6 



closely to the particles, but they are themselves, through 

 internal cohesion and surface tension, tenaciously strong; 

 and thus they are brought into a state comparable with 

 stretched rubber. On the other hand, the water molecules 

 are extremely mobile within the films, as if they were the 

 best ball bearings. From this combined tenacity and 

 mobility of the films, it results that when water is with- 

 drawn from any part of the soil, whether by root hairs or 

 by evaporation, the films directly affected 

 draw upon the others with which they are 

 connected, and these upon others, so that 

 the draft is thus made over a considerable 

 distance. Hence a plant is not dependent, 

 for its water supply upon the soil with 

 which its roots are in actual contact, but 

 can draw from a far wider area. This ex- 

 plains why a house plant dries out the soil 

 of the pot uniformly ; how Cactus and other 

 desert plants draw from great areas, growing 

 well spaced apart ; and why deep homogene- 

 ous soils, like those of the prairies, supply 



FIG. 170. A , -r-i , i 



root hair in the water so evenly to crops. Furthermore, since 

 soil, showing its fa e wa ter films have in general the same 



intimate contact . . , ,, ,. , , ., 



with soil par- thickness regardless of the size of the soil 

 tides; x 240 particles, a fine soil can retain more water 



(about). (After . , . . . , , , , . 



Strasburger.) than a coarse one, which is why clay holds 



more water than sand. 



AIR. This forms the third in abundance of the constituents 

 of ordinary soils, and is the source of the indispensible oxygen 

 for the respiration of most roots. It fills the irregular spaces 

 not occupied by water between the rock particles (Fig. 169) and 

 is ordinarily continuous with the atmosphere above ground. 

 In places of permanent hydrostatic water, like swamps, the 

 air is excluded, and only such plants can there live as have 

 large air passages to the roots from the leaves, or are able to 

 absorb dissolved oxygen directly into their submerged bodies 



