MOVEMENTS OF PLANTS. 651 



of the cell-contents, such as zoospores and spermatozoids, is generally 

 immediately resultant from the movement of cilia existing upon their 

 surface ; and the same statement applies to the locomotion of the " cell- 

 families," which form the representatives of species in the Volvocineae. 

 The locomotion of the Oscillatoriacese and Diatomacese, however, does 

 not appear to "be effected through the agency of cilia ; at all events no 

 such structure can "be detected with our present means of observation. 



The ciliary motion of the unicellular plants and reproductive bodies 

 is, although inexplicable, a more general phenomenon of life than the 

 movement of the Diatomacese &c., where these organs are not detected. 

 Some authors incline to regard the motion of Diatomacese as dependent 

 upon osmotic currents, resulting from the interchange of matter between 

 the cell-contents and the surrounding water. 



Movements from Changes in Hygroscopic State. Movements of various 

 kinds, more or less mechanical in appearance, take place in the higher 

 plants through the power of imbibition and the elasticity of their tissues ; 

 these movements are generally immediately produced by stimuli of vari- 

 ous kinds disturbing the equilibrium in the tissues. 



Great difference in the power of imbibition exists in different kinds of 

 cellular tissue collenchymatous tissue, for example, swelling out when 

 wetted, and contracting when dried, far more than woody structures. 

 Experiments have shown that the degrees of expansion and contraction 

 vary in different plants and tissues of plants in a range from yg 3 ^- to ^ the 

 diameter of the cells. In cases of great contraction a wrinkling of the 

 cell-membrane is generally involved. 



All living cell-membranes possess a certain degree of elasticity; and 

 consequently a certain amount of difference of dimensions is dependent 

 upon the degree of tension or turgescence of the cell from the presence 

 of fluid contents. Cells in which osmotic processes are going on are 

 constantly in a state of greater or less tension. 



The expansion of cellidar tissue through turgescence, permitted by the 

 elasticity of the membranes, appears to have a much smaller range than 

 the expansion by simple imbibition. The experiments of Unger and of 

 Brucke give a range of -^ to -J. 



It is evident that the elasticity of parenchymatous tissues must be 

 capable of exerting influence on the position, form, and direction of the 

 organs of which they form part. Supposing a tissue to be uniformly 

 developed, its expansion through turgescence need not alter the general 

 form, nor the relative position of the parts ; but if unequal endosmose 

 take place in different parts, causing disturbance of the equilibrium of 

 turgescence, curvature and distortion must ensue. Again, if an organ is 

 composed of regions in which the tissues differ in degree of elasticity, it 

 may suffer a disturbance of the equilibrium of turgescence still more 

 readily ; and this is probably the cause of most of the automatic move- 

 ments of organs of plants, as more fully explained under the head of 

 Tension (p. 609). 



Dehiscence of Fruits etc. This is usually the result of a greater degree 

 of turgescence in some cells than in others and ultimate rupture, and it 

 may also be due to irregularity in the process of drying, whereby cer- 

 tain layers contract more forcibly than others. The different anatomical 



