550 ORGANOLOGY. 



are first of all torn on the external surface, then separate themselves 

 from the point towards the basis as far as about the middle, curving 

 themselves outwards. A complete separation of the valves, and a 

 perfect tearing back, 'would, in consequence of the structure, here no 

 doubt take place, if, firstly, the cells of the suture were not thicker below *, 

 and capable of resisting the tension ; and, secondly, if the very thick 

 and tough partition-wall was not placed upon the centre of the valves, 

 which, like a prop, resists their curvature, and which .last action is still 

 further aided by the two longitudinal ribs, which project upon the 

 external side of each valve. 



The different movements of this kind are almost invariably produced 

 by the co-operation of the three phenomena here explained. It cannot 

 be expected that I should explain all possible cases, for which purpose 

 the necessarily accurate anatomical facts are still wanting. Every one 

 will easily be able to apply the above conditions to individual cases ; as 

 an instance, let us take the tearing of the capsule in Aspidium filix mas. 

 The capsule is flat, almost lenticular. A row of cells, commencing on 

 the one side from the stipes, forms round the greatest part of the cir- 

 cumference an imperfect ring, leaving on the other side a vacant space, 

 about one-sixth of the circumference. The cells are almost parallelo- 

 pipedical, and their walls towards the cavity of the capsule, where they 

 mutually touch each other (not towards the sides and exterior), are very 

 much thickened. The lateral walls, which pass into each other at the 

 before-mentioned one-sixth of the circumference, consist of very flat and 

 extremely thin-walled cells, The thicker and tougher walls of the cells 

 of the ring are but slightly, or not at all, changed by the process of 

 drying, but the thinner walls of the same cells are so. On the evapo- 

 ration of the fluid in the cells, they first contract in a somewhat elastic 

 manner, and thereby shorten the distance between the extreme end of 

 the thick walls, and thus of the whole external circumference of the 

 ring ; but as the moisture continues to evaporate without being supplied 

 with an equal quantity by air, the thin walls are pressed in by the atmo- 

 spheric pressure, and the contraction of the external circumference of 

 the ring is thus still more considerably increased. The internal circum- 

 ference, consisting of cells with thickened walls, remains unchanged, but, 

 through the contracted side walls that act as a rectangular lever, it 

 receives a tendency to straighten itself. This tension only continues as 

 long as the thin-walled cells at the last one-sixth of the circumference 

 are capable of resisting the expanding power ; as soon as the tension 

 becomes more powerful, they burst into a transverse slit, and the capsule 

 is opened. The progress of this process is quite similar in the teeth of 

 the capsules of the Mosses. 



215. The second kind of movements are seen in living parts 

 of plants during vigorous vegetation, and depend probably on the 

 distribution of the sap, and upon the elastic expansion of the in- 

 dividual cell-membranes. The facts, however, connected with this 

 subject are as yet too little known to admit of a clear explanation. 

 The following varieties of movements may be distinguished : 



* According to this, we can beforehand, by means of anatomical examination of the 

 cells, determine, in valves that are not completely separated, how far separation of the 

 valves will take place. 



