, BOTANY OF THE LIVING PLANT 



splits into two halves, each corresponding to one of its two 

 carpels, and each containing a seed. Each half is practically equiva- 

 lent to an achene (Fig. 490, vi -> Appendix A, p. 638). 



To release the seeds as separate bodies is one thing, to disseminate 

 them is a different proposition. By various modifications of the 

 ripening pistil this second end may also be secured, sometimes by the 

 activity of the pistil itself, but oftener through the medium of external 

 agencies. The plant itself may disperse its seeds by means of various 

 mechanical arrangements, which often result from the sudden release 

 of strains set up in the carpellary walls as they dry in the process of 

 ripening. An instance is seen in the common Hairy Cress (Cardamine 



kirsuta). Its fruit is the usual siliqua of the 

 Crucifers ; but the lateral valves, on splitting 

 from their base, curl so sharply upwards that 

 the seeds are forcibly thrown out (Fig. 101). 

 The method of the Geranium is similar, 

 though differing in detail. Here only one 

 seed is ripened in each of the five carpels. 

 By splitting from the base, and sudden curling 

 of the carpel upwards, the relatively large 

 seed is slung out at a distance (Fig. 243). 

 One of the largest and most effective of these 

 explosive fruits is that of the Sand-box Tree 

 (Hum, Fig. 102). It is composed of 12 to 

 18 woody carpels, each containing one seed 

 about I inch across. As the fruit hangs ripe 

 upon the tree, the carpels suddenly split apart, 

 and their woody shells take a twisted form, thus relieving the previous 

 strains. By the sudden change the seeds may be thrown to a distance 

 of some thirty yards from the tree. Another method is seen in the 

 tricarpellary capsule of the Violet. When ripe the three carpels 

 separate, and each boat-like carpel then presses its margins laterally 

 together as it dries, and pinches the smooth seeds, which are thus shot 

 out to a distance. Such examples illustrate the mechanical methods 

 of dispersal seen in dry fruits. 



In some pulpy fruits the principle of the squirt is used. The fruit 

 of the squirting Cucumber (Ecb allium) hangs pendent as it ripens, 

 and its outer pericarp is kept tense by the semifluid contents. When 

 ripe it breaks away at the stem, opening a basal pore. As it drops 

 the contents are sprayed out of it, seeds and all, to a considerable 

 distance, scattering as they fall. 



Fig. 243. 



Sling-Fruit of Geranium. 

 (After Figuier.) 



