124 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



the innermost consists of large turgid cells. When ripe, especially 

 if touched, the wall of the fruit suddenly separates into five segments 

 that curl inwards violently {cf. Fig. 28, C), shooting out the seeds 

 — in some species for fully 20 feet. In the Wood-sorrels (Oxalis 

 spp.) the ripe fruit suddenly splits lengthwise or by lateral slits 

 when touched, shooting out the mucilage-covered seeds. 



In many fruits the explosion that leads to a forcible ejection of 

 seeds is caused by stresses set up on drying. The audible cracking 

 of the pods of some members of the Pea family (Leguminosae) is of 

 this nature, the two valves of the pods (for example, when drying 

 in the sun) suddenly separating with often a violent spiral twisting, 

 and forcibly ejecting the hard and smooth seeds. Familiar European 

 examples of this are furnished by the Gorses {Ulex spp.). The 

 action is due to a hard layer of strongly thickened, elongate cells 

 lying transversely, and to which the softer tissues offer little resist- 

 ance. The distances to which seeds are shot by these means vary 

 greatly, but in some instances are said to exceed 40 feet and at 

 least rival the ejections of Hura and Hevea. Thus the turgor- and 

 drying-induced methods may be about equally effective. 



Many species of the familiar genus Viola, including some wild 

 Violets, in which the fruit splits into three boat-shaped valves 

 (Fig. 28, E), shootout their seeds for up to 15 feet as a result of unequal 

 drying of the layers of the fruit wall. This drying causes a curving 

 of the sides of the valves and the consequent pressing of the glossy 

 seeds together — until they ' pip out ', one after another, being often 

 further dispersed by rainv.ash. Also dispersed on explosion of the 

 hard ripe fruit are the seeds of almost all members of the family 

 Acanthaceae — sometimes to nearlv 30 feet from the parent — and 

 those of Claytonia, Montia, some Phloxes, and the Witch-hazels 

 {Hamamelis spp.). In the last instance the drying fruits may exert 

 such pressure on the seeds that these are discharged, like miniature 

 bullets, to distances of up to 40 feet — again rivalling Hiira and Hevea. 



In many members of the Geranium and Stork's-bill family 

 (Geraniaceae) the fruit suddenly splits into strips which curl up and 

 act as slings (Fig. 28, F) to throw out the seeds, which may travel 

 as much as 20 feet. Some fruits are also effectively dispersed by 

 mechanical propulsion, including those of Flat-figs {Dorstenia spp.) 

 which are embedded in the large fleshy receptacle that shrinks on 

 drying, setting up pressures which lead to the tiny fruits being 

 forcibly ejected. The spores of many Ferns are well known to 

 be discharged forcibly into the air by the springing backwards of 



