A part or organ may be called sensitive, when its irritation excites movement in an adjoining part.  Now it has been shown in this chapter, that the tip of the radicle of the bean is in this sense sensitive to the contact of any small object attached to one side by shellac or gum-water; also to a slight touch with dry caustic, and to a thin slice cut off one side.  The radicles of the pea were tried with attached objects and caustic, both of which acted.  With Phaseolus multiflorus the tip was hardly sensitive to small squares of attached card, but was sensitive to caustic and to slicing.  The radicles of Tropaeolum were highly sensitive to contact; and so, as far as we could judge, were those of Gossypium herbaceum, and they were certainly sensitive to caustic.  The tips of the radicles of Cucurbita ovifera were likewise highly sensitive to caustic, though only moderately so to contact.  Raphanus sativus offered a somewhat doubtful case.  With Aesculus the tips were quite indifferent to bodies attached to them, though sensitive to caustic.  Those of Quercus robur and Zea mays were highly sensitive to contact, as were the radicles of the latter to caustic.  In several of these cases the difference in sensitiveness of the tip to contact and to caustic was, as we believe, merely apparent; for with Gossypium, Raphanus, and Cucurbita, the tip was so fine and flexible that it was very difficult to attach any object to one of its sides.  With the radicles of Aesculus, the tips were not at all sensitive to small bodies attached to them; but it does not follow from this [page 192] fact that they would not have been sensitive to somewhat greater continued pressure, if this could have been applied.

The peculiar form of sensitiveness which we are here considering, is confined to the tip of the radicle for a length of from 1 mm. to 1.5 mm.  When this part is irritated by contact with any object, by caustic, or by a thin slice being cut off, the upper adjoining part of the radicle, for a length of from 6 or 7 to even 12 mm., is excited to bend away from the side which has been irritated.  Some influence must therefore be transmitted from the tip along the radicle for this length.  The curvature thus caused is generally symmetrical.  The part which bends most apparently coincides with that of the most rapid growth.  The tip and the basal part grow very slowly and they bend very little.

Considering the widely separated position in the vegetable series of the several above-named genera, we may conclude that the tips of the radicles of all, or almost all, plants are similarly sensitive, and transmit an influence causing the upper part to bend.  With respect to the tips of the secondary radicles, those of Vicia faba, Pisum sativum, and Zea mays were alone observed, and they were found similarly sensitive.

In order that these movements should be properly displayed, it appears necessary that the radicles should grow at their normal rate.  If subjected to a high temperature and made to grow rapidly, the tips seem either to lose their sensitiveness, or the upper part to lose the power of bending.  So it appears to be if they grow very slowly from not being vigorous, or from being kept at too low a temperature; also when they are forced to germinate in the middle of the winter. [page 193]