* See the remarks in Frank’s ‘Die wagerechte Richtung von Pflanzentheilen’ (1870, pp. 90, 91, etc.), on natural selection in connection with geotropism, heliotropism, etc. [page 571]

touched seems to be independent of its revolving or circumnutating movement.  This is best shown by the part which is the most sensitive to contact, circumnutating much less than the lower parts, or apparently not at all.*

Although in these cases we have no reason to believe that the movement depends on modified circumnutation, as with the several classes of movement described in this volume, yet the difference between the two sets of cases may not be so great as it at first appears.  In the one set, an irritant causes an increase or diminution in the turgescence of the cells, which are already in a state of change; whilst in the other set, the irritant first starts a similar change in their state of turgescence.  Why a touch, slight pressure or any other irritant, such as electricity, heat, or the absorption of animal matter, should modify the turgescence of the affected cells in such a manner as to cause movement, we do not know.  But a touch acts in this manner so often, and on such widely distinct plants, that the tendency seems to be a very general one; and if beneficial, it might be increased to any extent.  In other cases, a touch produces a very different effect, as with Nitella, in which the protoplasm may be seen to recede from the walls of the cell; in Lactuca, in which a milky fluid exudes; and in the tendrils of certain Vitaceae, Cucurbitaceae, and Bignoniaceae, in which slight pressure causes a cellular outgrowth.

Finally it is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals.** With plants an

* For the evidence on this head, see the ’Movements and Habits of Climbing Plants,’ 1875, pp. 173, 174.

** Sachs remarks to nearly the same effect:  “Dass sich die le-[[page 572]] bende Pflanzensubstanz derart innerlich differenzirt, dass einzelne Theile mit specifischen Energien ausgerüstet sind, ähnlich, wie die verschiedenen Sinnesnerven des Thiere” (’Arbeiten des Bot.  Inst. in Würzburg,’ Bd. ii. 1879, p. 282). [page 572]

astonishingly small stimulus suffices; and even with allied plants one may be highly sensitive to the slightest continued pressure, and another highly sensitive to a slight momentary touch.  The habit of moving at certain periods is inherited both by plants and animals; and several other points of similitude have been specified.  But the most striking resemblance is the localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves.  Yet plants do not of course possess nerves or a central nervous system; and we may infer that with animals such structures serve only for the more perfect transmission of impressions, and for the more complete intercommunication of the several parts.