CHAP. XII. CONCLUDING REMARKS. 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, circum- 

 nutating 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 cir- 

 cumnutation, 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 Bignoniacese, 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 uncon- 

 sciously by the lower animals.f With plants an 



* For the evidence on this pp. 173, 174. 



head, see tl}e ' Movements and f Sachs remarks to nearly the 



Habits of Climbing Plants,' 1875, same effect : " Dass sich die le- 



