MOVEMENTS DUE TO SWELLING, ETC. 



4'3 



bend the pod, the external epidermis is able to aid in the process, as Steinbrinck 

 (1873, p. 17) has suggested. The epidermal cells are elongated and traverse the 

 fibres, so that it is obvious that a differential contraction must occur between 

 them. In relation to what follows it is necessary to emphasize the fact that 

 twistings must result from the reactions between epidermis and fibres, since 

 differences such as those found by Zimmermann do not occur among the fibres 

 themselves, all the fibres behaving exactly alike. 



Of even greater interest is the spiral coiling exhibited by the lower parts 

 of the awns of Erodium (Fig. 123, A), for there the twisting takes place 

 obliquely to the long axis of the fibres of which the awn is composed — for 

 the epidermis and parenchyma need not be taken into account, having no power 

 of hygroscopic movement. The thick-walled 

 fibres are differentiated into four layers which 

 gradually merge into each other from without 

 inwards (Steinbrinck, 1895) : — 



1. A layer of fibres with transversely 

 placed pits, which when isolated bend out- 

 wards only slightly on drying. 



2. Fibres provided with pits which lie 

 transversely or are slightly tilted upwards to 

 the right on the outer walls and with pits on 

 the inner walls tilted upwards considerably to 

 the left. The whole layer and each individual 

 fibre when separated from the other layers 

 twists on drying just as the whole awn does. 



3. Fibres with pores arranged longitudi- 

 nally; these fibres on drying do not bend at all. 



4. A layer of fibres which when isolated 

 and dried twists to the right, i. e. in the oppo- 

 site direction to that of the awn as a whole. 



It wiU be obvious from what has been 

 said that to the second layer only or to that 

 in opposition to the other layers may be 

 attributed the twisting of the awn. Every 

 individual cell in it endeavours to twist on its 

 own account, and it is not difficult to see how 

 the twisting as a whole may be explained if we 

 compare the individual fibres of this layer with 

 the pods of the Papilionaceae. The transverse 

 or feebly oblique pores directed to the right on 

 the outer walls of these cells indicate to us 

 which is the long axis of the contraction-ellip- 

 soid ; they correspond in their deposition to the epidermal cells of the legume. 

 As regards the inner wall, the axis of the ellipsoid, as in the legume, lies almost 

 transversely to that of the outer walls. It is easily intelligible not only how 

 the second layer as a whole twists, owing to the efforts to twist of the individual 

 cells, but also how the first and third layers merely intensify this twisting. As a 

 matter of fact, it has been observed that the second layer twists whether in 

 conjunction with the first or the third or with both, and, finally, the fourth layer 

 must be added whose endeavours to twist in the reverse direction are completely 

 neutralized by the others. 



The last-mentioned layer behaves differently from the others for other 

 reasons. In layers 1-3 we have to deal with an antagonism between fiat inter- 

 secting plates, i. e. between layers whose axes of contraction intersect each other 

 (Steinbrinck, 1888). These plates are variously distributed :— (a) they occupy 



Fig. 123. Portion of the fruit of Erodium. 

 A, in the dry state ; B, in the wet con- 

 dition. From the Bonn Textbook. 



