SEEDS AND SEEDLINGS. 27 



The water passes round the edge of the cotyledons, into the 

 space between them, and into the radicle-pocket. Water 

 also enters by osmosis through the seed-coat, and at the 

 same time part of the solid matter contained in the young 

 plant passes out, in solution, into the water. This substance 

 which escapes from the seed is largely cane-sugar in the case 

 of the Broad Bean. When the cargo of a wheat-carrying 

 ship becomes damaged by water on a voyage, the weight of 

 the Wheat when dried again is found to have suffered con- 

 siderable loss by sugar passing out into the water. We shall 

 see later how the Bean seed and Wheat grain come to contain 

 sugar after soaking in water and beginning to germinate. 



SO. Capillarity. The free surface of water (and other liquids) 

 behaves much as though it were covered by an elastic membrane 

 (surface film). It is this surface tension that draws the raindrop 

 into the form of a sphere as it falls through the air, and that causes 

 water to form into spheres on a dusty floor or on the leaves of many 

 plants (or on any surface which is not wettable by water). The cause 

 of surface tension is the attraction of the particles (molecules) of water 

 for each other. This attraction extends throughout the liquid, but 

 only the molecules at the surface show its influence, because it is only 

 these that are not pulled evenly in all directions by water-molecules 

 on every side. 



"#"hen open glass tubes, having a very small bore, are held vertically 

 in a liquid, there is always a difference between the level of the liquid 

 inside the tube and that of the liquid outside. If a glass tube of this 

 kind be placed in water, the level of the water in the tube will be above 

 that of the water outside, and the surface of the water in the tube will 

 be concave. In the case of mercury, the level inside will be below that 

 outside, and the surface will be convex. The finer the tube, the more 

 marked will the result be. The liquid only ascends when it is capable 

 of wetting the sides of the tube. 



Hence we have two kinds of capillary action in one the liquid wets 

 the tube, ascends, and has a concave surface, in the other it does not 

 wet the tube, descends, and has a convex surface. We need only deal 

 with the capillary ascent of water, which is of importance in plant-life. 

 If the tube (supposed circular in cross section) is 1 in. in diameter, the 

 water rises 0'054 in. ; if O'l in. in diameter, the water rises 0*54 in., and 

 so on. In a tube O'OOl in. in diameter the water rises 54 ins. 



This capillary action is closely connected with the surface tension 

 of the water, and it is therefore natural that the total lifting force 

 which supports the column of water should be proportional to the 

 circumference of the tube. The circumferences of tubes increase 

 in the same proportion as their diameters (circumference of a circle 

 = 3*14 x diameter), hence a tube O'l in. in diameter will lift above the 

 water-level ten times as much water as a tube O'Ol in. in diameter. 



