ROOTS ai 
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. Thenruna 
slender needle into the 
root where it enters 
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? 
Fie. 26.— A Sprouting Windsor Bean pushing its 
Root-Tip into Mercury. 
s, seed ; 7, root ; w, layer of water ; m, mercury. 
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 of 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 
