Little squares of card were affixed with shellac on two occasions to the tips of 22 young and short secondary radicles, which had been emitted from the primary radicle whilst growing in water, but were now suspended in damp air.  Besides the difficulty of attaching the squares to such finely pointed objects as were these radicles, the temperature was too high,—­varying on the first occasion from 72o to 77o F., and on the second being almost steadily 78o F.; and this probably lessened the sensitiveness of the tips.  The result was that after an interval of 8 h. 30 m., 6 of the 22 radicles were bowed upwards (one of them greatly) in opposition to gravity, and 2 laterally; the remaining 14 were not affected.  Considering the unfavourable circumstances, and bearing in mind the case of the bean, the evidence appears sufficient to show that the tips of the secondary radicles of the pea are sensitive to slight contact.

Phaseolus multiflorus:  Sensitiveness of the apex of the Radicle.—­ Fifty-nine radicles were tried with squares [page 164] of various sizes of the same card-like paper, also with bits of thin glass and rough cinders, affixed with shellac to one side of the apex.  Rather large drops of the dissolved shellac were also placed on them and allowed to set into hard beads.  The specimens were subjected to various temperatures between 60o and 72o F., more commonly at about the latter.  But out of this considerable number of trials only 5 radicles were plainly bent, and 8 others slightly or even doubtfully, from the attached objects; the remaining 46 not being at all affected.  It is therefore clear that the tips of the radicles of this Phaseolus are much less sensitive to contact than are those of the bean or pea.  We thought that they might be sensitive to harder pressure, but after several trials we could not devise any method for pressing harder on one side of the apex than on the other, without at the same time offering mechanical resistance to its growth.  We therefore tried other irritants.

The tips of 13 radicles, dried with blotting-paper, were thrice touched or just rubbed on one side with dry nitrate of silver.  They were rubbed thrice, because we supposed from the foregoing trials, that the tips were not highly sensitive.  After 24 h. the tips were found greatly blackened; 6 were blackened equally all round, so that no curvature to any one side could be expected; 6 were much blackened on one side for a length of about 1/10th of an inch, and this length became curved at right angles towards the blackened surface, the curvature afterwards increasing in several instances until little hooks were formed.  It was manifest that the blackened side was so much injured that it could not grow, whilst the opposite side continued to grow.  One alone out of these 13 radicles became curved from the blackened side, the [page 165] curvature extending for some little distance above the apex.