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.
(3.) After one day slight deflection, but the cauterised mark was so faint that the same side was again touched with caustic. In four days from the first touch deflection amounted to 78o, which in an additional day increased to 90o.
(4.) After two days slight deflection, which during the next three days certainly increased but never became great; the radicle did not grow well and died on the eighth day.
(5.) After two days very slight deflection; but this on the fourth day amounted to 56o from the perpendicular and from the cauterised side.
(6.) After three days doubtfully, but after four days certainly deflected from the cauterised side. On the fifth day deflection amounted to 45o from the perpendicular, and this on the seventh day increased to about 90o.
(7.) After two days slightly deflected; on the third day the deflection amounted to 25o from the perpendicular, and this did not afterwards increase.
(8.) After one day deflection distinct; on the third day it amounted to 44o, and on the fourth day to 72o from the perpendicular and the cauterised side.
(9.) After two days deflection slight, yet distinct; on the third day the tip was again touched on the same side with caustic and thus killed.
(10.) After one day slight deflection, which after six days increased to 50o from the perpendicular and the cauterised side.
(11.) After one day decided deflection, which after six days increased to 62o from the perpendicular and from the cauterised side.
(12.) After one day slight deflection, which on the second day amounted to 35o, on the fourth day to 50o, and the sixth day to 63o from the perpendicular and the cauterised side.
(13.) Whole tip blackened, but more on one side than the other; on the fourth day slightly, and on the sixth day greatly deflected from the more blackened side; the deflection on the ninth day amounted to 90o from the perpendicular.
(14.) Whole tip blackened in the same manner as in the last case: on the second day decided deflection from the more blackened side, which increased on the seventh day to nearly 90o; on the following day the radicle appeared unhealthy.
(15.) Here we had the anomalous case of a radicle bending [page 174] slightly towards the cauterised side on the first day, and continuing to do so for the next three days, when the deflection amounted to about 90o from the perpendicular. The cause appeared to lie in the tendril-like sensitiveness of the upper part of the radicle, against which the point of a large triangular flap of the seed-coats pressed with considerable force; and this irritation apparently conquered that from the cauterised apex.]
These several cases show beyond doubt that the irritation of one side of the apex, excites the upper part of the radicle to bend slowly towards the opposite side.