364 



A TEXTBOOK OF BOTANY [Ch. VII, 3 



plants are accomplished bjf man himself, who carries plants 

 dehberately all around the world for his own purposes, 

 and also spreads them accidentally through his commerce 

 and travel. Much of his deliberate dispersal has effect 

 only so long as his watchful care persists, for without it the 

 most of his carefully grown crops would soon be exter- 

 minated by the return of the native vegetation. 



6. Projection by spring-release. In the dissemination of 

 plants by aid of winds, water currents, and animals, the 



fruits and seeds are 

 wholly passive, and 

 the results are secured 

 by the presence of 

 structures which make 

 the dissemination an 

 inevitable result of the 

 natural and ordinary 

 opei-ations of those 

 agencies. There is, 

 however, one waj- in 

 which plants eiTect an 

 active dissemination 

 by forces developed 

 within themselves, — 

 namely, in the hurUng 

 of seeds through the 

 air liy the action of suddenly released spring mechanisms. 

 Thus in Violets (Fig. 264), the pod so ripens that the carpels 

 press harder and harder upon the smooth seeds held in 

 an angle between them, until suddenly the pressure over- 

 comes the friction and the seeds are shot to a distance, 

 much as one may shoot a smooth bean from between the 

 pressed fingers. Again, in the ^'etches, bands of tissue in 

 the pod so ripeni under tension as to bring sucli a strain 

 on the sutures of dehiscence that they suddenly rupture 

 and shoot the seeds forth in everj- direction (Fig. 265)» 



Fig. 264. — Seed pods of a Violet, forcibly 

 projecting the seeds. (From Kerner.) 



