262 FRUITS AND SEEDS. 



291. Explosive Fruits. Some fruits show active move- 

 ments by which the seeds are scattered or flung out suddenly. 

 These movements often depend upon extreme turgidity in 

 some part of the fruit e.g. in Squirting Cucumber and Bal- 

 sams or of the seed itself. In some Balsams the fleshy 

 capsules have swollen and stretched walls, so that a slight 

 disturbance causes the capsule to burst, and the seeds are 

 thrown to a distance of several feet. 



In some dry fruits the ejecting mechanism depends upon 

 tensions set up by the drying of the fruit wall. In Pansies 

 and most Violets the flower-stalk grows up erect after pol- 

 lination has occurred, and the ripe capsule splits down, mid- 

 way between the three lines of seeds, into three boat- shaped 

 pieces, and as these dry they contract and squeeze the seeds 

 together; the pressure causes the polished and slippery 

 seeds to be flicked out forcibly and thrown to a consider- 

 able distance (sometimes as much as six feet in G-arden 

 Pansies) . 



In G-eranium (Fig. 93, D) the carpels split apart, and the 

 style splits off a slender thread attached to each carpel ; these 

 threads suddenly curl upwards and outwards, so as to throw 

 the seeds out. The ripe pods of G-orse, Broom, Lupin, etc., 

 suddenly burst open, the two valves becoming twisted and 

 the seeds scattered. More or less similar explosive mechanisms 

 occur in various other plants. 



(a) In ripe fruits of Wood Sorrel notice that some of the chambers 

 may be empty ; if so, find the slit through which the seeds have 

 escaped. This suggests that the seeds are in some way forced out of 

 the chambers, the longitudinal slit running down the middle of the 

 chamber-wall closing up again at once. How do the seeds escape ? 

 Try squeezing ripe fruits between the fingers until you succeed in 

 causing seeds to spring out. How far may a seed be jerked away ? 

 By carefully examining (1) a ripe fruit with the seeds still inside, 

 (2) a fruit from which the seeds have escaped, (3) the seed itself 

 both before and after its escape, you should be able to discover 

 that the seed has a thick fleshy extra coat (aril) covering it like a 

 cup, and that this coat, by suddenly turning inside out, causes 

 the seed to be shot out of the fruit through the slit in the chamber- 

 wall. 



(&) Examine and sketch the fruits of Violet, Pansy, Geranium, 

 Gorse, Broom, Lupin, Box-tree, Spurge, Hairy Bittercress. Try to 

 find out how the seeds are ejected in each case. 



