650 PHYSIOLOGY. 



Sect. 3. MOVEMENTS OP PLANTS. 



The absence of the phenomena of motion was formerly ima- 

 gined to constitute one of the distinguishing characters of plants 

 as contrasted with animals ; but, in fact, members of the Vegetable 

 Kingdom not only exhibit partial movements of their internal or 

 external structures, but in some cases the entire organism has the 

 power of locomotion. 



Exciting causes. The movements of different plants are de^ 

 pendent upon very diverse causes, some of them being entirely 

 mechanical, and due to physical affections of the tissues by the 

 conditions of the surrounding atmosphere, and to alternate con- 

 ditions of turgescence and exhaustion ; others are mechanical in 

 appearance, but excited by causes which simple physical laws will 

 not explain ; while a third kind depend upon the contractile quality 

 of the protoplasmic substances, which gives rise to automatic move- 

 ments comparable to those of the lower animals and to the ciliary 

 motion found in particular tissues of the highest animals. 



To the first of these classes belong such phenomena as the bursting of 

 seed-vessels, anthers, &c., attributable in part to the hygroscopic condi- 

 tions of the tissues, which, possessing unequal power of imbibition and 

 unequal elasticity, are torn apart or curved in various ways by unequal 

 contractions and expansions, caused by access or abstraction of moisture. 

 These are cases of what is called in common language "warping," and 

 can scarcely be regarded as vital phenomena, being definite modes of 

 destruction of dead structures, resulting from special structural condi- 

 tions. 



The second class of movements are those exhibited periodically, or 

 under special stimulus, by the external organs of plants, such as the 

 " sleeping " and " waking " of leaves and flowers, and the movement of 

 the Sensitive Plants, &c. To these may perhaps be added the less striking, 

 but not less enigmatical, movements which cause the twining condition of 

 stems &c. 



The movements of the third class are those already adverted to in 

 former sections, where the protoplasmic matter of cell-contents, or free 

 bodies, such as zoospores, spermatozoids, or even perfect individuals, such 

 as Desmidiece, Oscillatoriacese, exhibit temporary or permanent power of 

 locomotion. Of this character also are the protrusions observed by Mr. 

 Francis Darwin of fine threads of protoplasm from the pear-shaped glands 

 of the common Teazel (JDipsacus). These threads have the power of 

 contracting strongly, and the contraction is further induced by acids, by 

 induced electric currents, by heat, and other agents. 



The rotation of the protoplasmic cell-contents (p. 549) doubtless depends 

 upon (imperfectly understood) causes similar to those which render many 

 of the simpler plants locomotive ; and the movements exhibit the closest 

 resemblance to those of some of the Protozoa in the Animal Kingdom 

 in particular, of Am&ba and its allies. The locomotion of free products 



