SLING-FRUITS. 837 



empty and rolled-up carpels, resembles a chandelier in shape (shown to right of 

 fig. 459 s). 



Those Violets which have aerial stems, such as Viola elatior (see fig. 459*), 

 develop capsular fruits, each of which resolves itself into three valves when it bursts 

 open. The valves are boat-shaped, and the marginal parts which form the sides of 

 the boats are thin, whilst the keels are very thick and swollen. Inside each boat, 

 near and parallel to the line of the keel, are two rows of seeds. The valves them- 

 selves have an exceedingly complex structure. A cross section through one of them 

 shows a layer of thin-walled parenchymatous cells, a layer of elongated curvilinear 

 cells, and a layer of broad, greatly thickened cells. The unequal desiccation of these 

 layers is the cause of the curving up of the lateral walls of the valves, which at last 

 approach so near to one another as to exercise considerable pressure on the seeds in 

 the middle. The result of this pressure is that the smooth seeds are shot out with 

 about the same force as is imparted to a cherry-stone when it is flicked to a distance 

 by the finger and thumb. The seeds are ejected in regular succession. The foremost 

 seed of the first carpel goes first, and the seeds at the opposite extremity are dis- 

 charged last. It is not till the first carj)el is quite empty that the second begins to 

 part with its seeds, and the third only comes into play when the second is finished. 

 The drawing together of the two sides of the valve always begins at the free 

 extremity of the valve, and lasts until all the seeds have been ejected. 



In many Mimosese, Csesalpinese, Papilionaceae, Sterculiacese, and Acanthacese 

 the seeds are expelled by means of a spiral torsion of the valves of the fruit at the 

 moment that the legume or capsule opens. The wall of the fruit of these plants 

 includes a soft succulent layer of thin-walled parenchymatous cells, and a hard layer 

 of strongly-thickened elongated cells, which run obliquely from one edge to the other 

 in each valve. The rupture of the fruit, and the spiral torsion of its valves at the 

 moment of their separation, depend upon these diagonal cells of the hard layer. 

 Each one of these cells winds itself into a spiral as it dries, and consequently the 

 entire layer undergoes a corresponding torsion. The tissues composed of thin-walled 

 cells, which are in connection with the hard layer, offer no resistance to the move- 

 ment, and the rotation is therefore so sudden and violent that the seeds contained in 

 the pod are projected to a distance. If the fruit is short, the valvular torsion is 

 confined to |-1 twists; if long, the spiral includes 2 or even 3 complete coils, and 

 the valves of the empty fruit are curled up like ringlets (e.g. Lotus corniculatus, 

 see p. 431, fig. 325 ^ and Orobus vernus, see fig. 459^). The force of projection 

 varies according to the thickness of the hard layer. In CastanosperinuTn australe, 

 where the pod- valves attain to a thickness of 5 millimetres, the sudden torsion causes 

 the expulsion of spherical seeds, measuring 3'5 centimetres in diameter, and weighing 

 16 grams. In these cases the valves of the fruit persist upon the fruit-stalks after 

 the ejection of the seeds, and herein lies the essential difference between them and 

 those expulsive fruits of which the carpels break away from the stalks with the 

 seeds. To this class of expulsive fruits belong also several Papilionacese, such as the 

 Dorycnium mentioned at the beginning of this section, and besides them the genus 



