MOVEMENTS DUE TO SWELLING, ETC. 415 



The inner tangential walls of the cells of the annulus are strongly thickened, 

 but the outer walls are unthickened, while on the radial walls the thickening is 

 gradually reduced from within outwards. When the sporangium is ripe, at 

 the region where the annulus ceases, or, to be more exact, where the cells of the 

 annulus cease to have thickened walls (Fig. 124, /, st), there occurs a rupture due 

 to the contraction of the annulus, so that the sporangium takes the form seen 

 in Fig. 124, 2, and the spores can thus escape. Obviously in consequence of the 

 loss of water from its constituent cells, the annulus contracts and bends slowly 

 outwards, and this contraction and bending may proceed so far that the 

 annulus again forms a circle, but now what was the inner face becomes 

 the outer. At this moment a new phenomenon appears, for with a sudden 

 snap the annulus springs back and once more assumes almost its original 

 position and form. It rebounds on its base with considerable force, and the 

 whole sporangium is thrown often several centimetres into the air, and in the 

 process the spores which still adhere to the capsule are ejected. Looking more 

 closely at the annulus during the opening of the sporangium we see that its 

 cells undergo a remarkable deformation. The water they contain evaporates, 

 and the cell cavities become smaller, the thin external walls become concave 

 inwards while the lateral walls approximate. The external outline of the 

 annulus thus becomes gradually shorter, and the inward curving of the ring is 

 thus simply explained. The curving in- 

 wards of the outer walls of the individual 

 cells of the annulus and the water they con- 

 tain proves clearly that this is not a case of 

 ' shrivelling '. 



Inquiry as to the factors which bring 

 about the ultimate deformation of the 

 cell whereby the outer wall approximates 

 to the base of the cell and the outer 

 corners of the radial walls come to touch 

 each other, shows us that we must in- 

 vestigate the cohesion of the imbibition 



Water Of the Cell and its adhesion tO Fig. 124. Closed (/) and open (^ sporangia 



the membrane (SXEINBRINCK, 1898, 1903). of the P>yP<>diaceae. /, lip-cells; a, annulus. 



In speaking of the movements of water 



in the plant we have already shown that the cohesive force of water is 

 very great. The adhesive force of water to the membrane is almost as great 

 inasmuch as the tension of several atmospheres is required to tear the water 

 particles apart from each other or from the wall. When evaporation commences, 

 therefore, the water in the cells must be under tension and the effect of that 

 tension is to produce deformation of the cell. If the cell-wall could not be de- 

 formed the water cohesion would soon be overcome and an air space would 

 appear in the interior of the cell, or a rupture would occur between the water 

 and the membrane, and air would at once enter. It was previously thought 

 that such movements as those seen in the fern annulus (and which we may term 

 cohesion movements, in contradistinction to those due to swelling and con- 

 traction) could only take place if the cell membrane were impermeable to air. 

 If that were the case their occurrence would be very restricted, and they could 

 not in any case take place in the fern sporangia since the outer walls of the 

 annulus cells are, as a matter of fact, quite permeable to air. The entry of air 

 into the interior of the cell is, however, rendered impossible at first, inasmuch 

 as each minute bubble of air must first of all overcome the adhesion of the water 

 to the cell-wall. 



The imbibition water in the annulus cells exercises in this way during 

 evaporation a vigorous pull on the walls, and stretches them elastically. Finally, 



