ROOTS 



57 



to a cork that is fastened with beeswax and resin mixture to the 

 side of a little trough or pan of glass or glazed earthenware. The 

 pan is filled half an inch or more with mercury, and on top of 

 the mercury is a layer 

 of water. The whole 

 is closely covered by 

 a large tumbler or a 

 bell-glass. Allow the 

 apparatus to stand un- 

 til the root has forced 

 its way down into the 

 mercury. Then run a 

 slender needle into the 

 root 'where it enters 



FIG. 26. A Sprouting Windsor Bean pushing its 

 Hoot-Tip into Mercury. 



s, seed ; r, root ; w, layer of water ; m, mercury, 

 the mercury (to mark 



the exact level), withdraw the root, and measure the length of 

 the part submerged in mercury. To see whether this part would 

 have stayed under by virtue of its own weight, cut it off and lay 

 it on the mercury. Push it under with a pair of steel forceps and 

 then let go of it. What does it do ? 



69. Discussion of Exp. XIX. By comparing the weights 

 of equal bulks of mercury and Windsor bean roots, it is 

 found that the mercury is about fourteen times as heavy 

 as the substance of the roots. Evidently, then, the sub- 

 merged part of the root must have been held under by 

 a force about fourteen times its own weight. Making fine 

 equidistant cross-marks with ink along the upper and the 

 lower surface o'f a root that is about to bend downward at 

 the tip, readily shows that those of the upper series soon 

 come to be farther apart, in other words, that the. root is 

 forced to bend downward by the more rapid growth of its 

 upper as compared with its under surface. 



70. Geotropism. The property which plants or their 

 organs manifest, of assuming a definite direction with 



