ON THE POWER OF MOVEMENT IN PLANTS. 149 



ments seemingly so, but which we cannot explain by physical 

 causes alone ; of these presently. 



In this Class are included the bursting of spore-cases in Cryp- 

 togams ; for instance, the rupture of the sporangia in Ferns, the 

 breaking away of the opercula in Mosses, and the unwinding of 

 the elastic elaters on the spores of Equisetums. Also the dehis- 

 cence, or bursting of anther-cells for the escape of the fertilizing 

 pollen, and the dehiscence of the Fruit in Flowering plants. 



All these are cases of ' warping,' so to speak, and are the 

 destruction of parts in a sense 'dead,' resulting from some intrinsic 

 structural conditions acted on by external physical phenomena. 

 In the cases of anthers, dehiscence is produced partly by pressure 

 of the pollen grains on the coats of the anther-lobes, causing partial 

 absorption of the latter, partly by special action of the fibrous cells 

 lining the anther. 



The main causes, however, of this class of movements are two : 

 varying power of imbibition of moisture, and varying degree of 

 elasticity in the tissues. These again are effected by the hygro- 

 scopic condition of the atmosphere ; nowhere is this better seen 

 than in the elaters of Eqidsetum^ where, first of all, the spore-case 

 splits at the proper time, and then the individual spores, whose 

 outer coat has furnished (by splitting in narrow strips, which become 

 detached) the said organs, uncoil their club-shaped elaters, which, 

 acted on by the moisture of the air, assist in the carrying of the 

 spores. Power of imbibition varies greatly, the degree of expan- 

 sion of cells due to this varying from one-thousandth to one-half 

 of the cell-diameter. 



Expansion due to turgidity varies through a smaller range than 

 that due to simple imbibition, the range being only from one-eight- 

 ieth to one-fifth of the diameter of any cell. If unequal absorption 

 of moisture takes place in various parts, distortion or curvature 

 comes about and often permanent rupture. Moreover, if different 

 degrees of elasticity should ensue, the equilibrium of turgidity is 

 still more displaced. These, together with the well-known con- 

 tractile powers of protoplasm and its power of motion from cell 

 to cell, will probably account for most of these movements. 



The dehiscence of seed-capsules or Fruits, be it valvular (that is, 

 longitudinal), or by pores, as in Poppy, or by a circular slit, the top 



VOL. v. M 



