154 DR. FRANCIS DARWIN ОМ THE MECHANISM ВУ WHICH 
The following measurements give the amount buried in three instances during a 
single drying :— 
L 
6.45 P.M.—9 mm. of seed projecting above surface. 
8.15 А.м.—7 mm. 5 
Amount buried 2 mm. 
ә 
15% 
11.30 А.М.- 192 mm. projecting above surface. 
1. 2 р.м.—10 mm. о b 
Amount buried 24 mm. 
TIE 
9 A.M.—10 mm. projecting above surface. 
11 А.М.-- 6 mm. 
33 РЕД 
Amount buried 4 man. 
In investigating the burial of the seed caused by drying, it must first be remarked that 
the plume of hairs with which the point is armed (see figs. 1 & 7) offers but little resist- 
ance to the entrance of the seed, but tends to prevent its withdrawal. Тһе bare sharp 
point ( p, figs. 1 & 7) is about 3 mm. in length; the hairs which form the plume are 
graduated, the smallest (1 mm.) being the nearest to the point, so that they tend to 
preserve the smallest amounts buried during the wetting process. Moreover the length 
of the whole plume, measured from the extreme tip of the seed, is only about 4 mm. ; and 
as we have seen that as much as 5 mm. may be buried in one wetting, it is quite con- 
ceivable that the seed can resist the uprooting tendency of the drying process. То 
understand how it can actually be buried deeper, we must examine more closely the act 
of rotation during drying. 
А vertical rod, which is fixed at the upper extremity, will as it twists on its axis, 
cause a simple rotation of its lower free extremity. But if the upper or fixed end is not 
vertically above the lower or revolving end, 4. e. if the rod is curved, there will, 1 
believe, be a tendency for the rotation to take place about the line joining the upper and 
F lower ends of the rod. The movement imparted to the point will 
then resemble the circular rocking motion which is employed when 
а stick is being thrust into the ground. Given pressure from above, 
a rotating vertical rod will make a cylindrical hole of continually 
increasing depth; but if this pressure is removed, it will merely 
7S continue to rotate in the hole already made. But in the case of à 
g n: 4 Stipa-awn (a curved rod twisting on its axis), even when the pres- 
P sure from above is removed the rocking* tends to continue the bury- 
ing process. Тһе point of the seed may be compared to a barbed point, like that in the 
woodeut. 
The"point P is supposed to be thrust into the ground, gg’ giving the level of the soil. 
* Whatever may be the explanation of this movement, it certainly occurs. 
