MOVEMENTS DUE TO SWELLING, ETC. 411 



desiccation sets in, must be due to the differences in their mode of deposition. 

 The cells contract relatively less in a longitudinal direction than trans- 

 versely, hence the outer sides of the segments contract more vigorously than 

 the inner sides, and the segments become concave outwardly. It is not, 

 however, essential that the elements of both the antagonistic layers should be, 

 as in Syringa, elongated and cross each other at 90 or less ; it is sufficient for 

 the purpose if one cell layer be composed of fibres while the other is formed of 

 isodiametric cells. Thus in the wall of the fruit of Veronica we find the 

 epidermal cells of the interior are thick-walled fibres, whilst externally there 

 lies a layer of parenchyma capable of contracting equally in all directions 

 (STEINBRINCK, 1878). Differential contraction between this layer and the inner 

 epidermis is greatest in the long axes of the latter elements, and must, therefore, 

 cause a bending outwards at right angles to the course of these elements. 



2. Differential contraction due to lamination of the cell-wall. As an example 

 of this type we may select the capsular teeth of Linaria (STEINBRINCK, 1891). 

 Fig. 120, /, shows a part of a median longitudinal section of that portion of the 

 tooth specially concerned in the contractile movement, that is, through the 

 inner epidermis and the sclerotic layer abutting on it. The cells figured are, 

 it is true, by no means isodiametric, nevertheless the important factor in the 

 bending is not the arrangement of the cells but the lamination of the cell-mem- 

 branes. The two cell-layers differ essentially in the way in 



which their cell-membranes are deposited. The lamina- 



tion of the inner epidermis is almost perfectly parallel to 



the long axes of the capsular teeth, and the same is true 



of the inner walls of the sclerotic layer. In the remaining 



portion of the latter one would expect a similar arrange- 



ment, in accordance with the usual principles of 



unilateral thickening, that is to say, a deposition of 



secondary thickening in layers tangential to the outer 



surface as in Fig. 120, 2. As a matter of fact, however, 



a glance at Fig. 120, /, shows that all the lamellae are laid 



down parallel to the horizontal walls . Since the maximum 



contraction, as we have already pointed out, occurs at 



right angles to the lamination, the greater part of the (1891). 



sclerotic layer contracts much more markedly in the long 



axis of the capsule than the inner epidermis and the inner wall of the sclerotic 



layer. Measurements made on the isolated layer exhibited a shortening in the 



former of 10 per cent., while in the latter the contraction was scarcely observable. 



3. Differential contraction due to striation in the cell-wall. We may briefly 

 characterize the last case we took by saying that in it the difference between the 

 shortest and a longer axis of the contraction-ellipsoid is made the most of. 

 In contrast to this we find cases in which the difference between the longest and 

 the average axis comes into play, the cases in which the bending is due to stria- 

 tion of the membrane, (a) The capsules of Campanula (STEINBRINCK, 1895) 

 open in the same way as those of Linaria, viz. by valves, but the histological 

 structure and the mechanism of opening is quite different in the two plants. 

 The sclerenchyma is absent from Campanula altogether, and the bending 

 is brought about by the parenchyma, and is due in part to the form of the cells, 

 the external layers being composed of short cells gradually increasing in length 

 inwards. Any bending must therefore take place so that the concavity is 

 external, in accordance with the principles already laid down. A second factor 

 is the striation of the cell- wall which expresses itself in the position of the pits. 

 The outer cells have their pits arranged transversely, while on the walls of the 

 cells of the succeeding layers the pits are laid down obliquely to the left and finally 

 longitudinally. Since, as we have seen, the long axis of the contraction-ellipsoid 



section through a valve of 



