EXPLOSIVE FRUITS; WINGED FRUITS 



439 



and if the ground slopes they may easily roll far enough to get 

 started in new places. 



413. Dispersal of seeds. Seeds are not infrequently scattered 

 by apparatus with the aid of which the plant throws them about. 

 More commonly, however, they depend upon other agencies, 

 such as wind, water, or animals, to carry them. Sometimes the 

 transportation of seeds is due to the structure of the seeds them- 

 selves, sometimes to that of the fruit in which they are inclosed ; 

 the essential point is to have transportation to a long distance 

 made as certain as possible, to avoid overcrowding. 



414. Explosive fruits. Some dry fruits burst open when 

 ripe in such a way as to throw their seeds violently about. 

 Interesting studies may 



be made, in the proper 

 season, of the fruits of 

 the common blue violet, 

 the pansy, the wild 

 balsam, the garden bal- 

 sam, the cranesbill, the 

 herb Robert, the witch- 

 hazel, the Jersey tea, and 

 some other common 

 plants. The capsule of 

 the tropical American 

 sand-box tree bursts open 

 when throughly dry with Fict - 

 a noise like that of a pis- 

 tol shot. The explosive force of fruits is derived from the fact 

 that some of their parts on drying are left in a state of ten- 

 sion, some layers of cells being compressed or stretched and 

 tending to readjust their position. 



415. Winged or tufted fruits and seeds. The fruits of the 

 ash, box elder, elm, maple (Fig. 160), and many other trees are 

 provided with an expanded membranous wing. Some seeds, as 

 those of the catalpa and the trumpet creeper, are similarly 



339. Fruits of linden, with a bract joined 

 to the peduncle and forming a wing 



