strongly bowed downwards after 8 h.  From the several foregoing facts, there can be no doubt that the cauterisation of the tip of the radicle of this Cucurbita on one side, if done lightly enough, causes the whole growing part to bend to the opposite side.  Raphanus sativus:  Sensitiveness of the apex of the Radicle.—­We here encountered many difficulties in our trials, both with squares of card and with caustic; for when seeds were pinned to a cork-lid, many of the radicles, to which nothing had been done, grew irregularly, often curving upwards, as if attracted by the damp surface above; and when they were immersed in water they likewise often grew irregularly.  We did not therefore dare to trust our experiments with attached squares of card; nevertheless some of them seemed to indicate that the tips were sensitive to contact.  Our trials with caustic generally failed from the difficulty of not injuring too greatly the extremely fine tips.  Out of 7 radicles thus tried, one became bowed after 22 h. at an angle of 60o, a second at 40o, [page 172] and a third very slightly from the perpendicular and from the cauterised side.

Aesculus hippocastanum:  Sensitiveness of the apex of the Radicle.—­Bits of glass and squares of card were affixed with shellac or gum-water to the tips of 12 radicles of the horse-chestnut; and when these objects fell off, they were refixed; but not in a single instance was any curvature thus caused.  These massive radicles, one of which was above 2 inches in length and .3 inch in diameter at its base, seemed insensible to so slight a stimulus as any small attached object.  Nevertheless, when the apex encountered an obstacle in its downward course, the growing part became so uniformly and symmetrically curved, that its appearance indicated not mere mechanical bending, but increased growth along the whole convex side, due to the irritation of the apex.

That this is the correct view may be inferred from the effects of the more powerful stimulus of caustic.  The bending from the cauterised side occurred much slower than in the previously described species, and it will perhaps be worth while to give our trials in detail.

[The seeds germinated in sawdust, and one side of the tips of the radicles were slightly rubbed once with dry nitrate of silver; and after a few minutes were allowed to dip into water.  They were subjected to a rather varying temperature, generally between 52o and 58o F. A few cases have not been thought worth recording, in which the whole tip was blackened, or in which the seedling soon became unhealthy.

(1.) The radicle was slightly deflected from the cauterised side in one day (i.e. 24 h.); in three days it stood at 60o from the perpendicular; in four days at 90o; on the fifth day it was curved up about 40o above the horizon; so that it had passed through an angle of 130o in the five days, and this was the greatest amount of curvature observed.

(2.) In two days radicle slightly deflected; after seven days [page 173] deflected 69o from the perpendicular and from the cauterised side; after eight days the angle amounted to nearly 90o.