920 



ECOLOGY 



is so sudden and violent that the seeds are expelled at the same time. 

 At dehiscence the seeds of the violet and lupine are shot out several 

 centimeters (sometimes nearly a meter), while those of the witch-hazel 

 are expelled much more violently, and may be scattered for several 

 meters. In the lupine the seeds are expelled spirally by reason of the 

 torsion of the valves (fig. 12 12). 



In Geranium the carpels separate from the central axis, coiling upwards and 

 discharging the seeds. In Hura crepitans the dehiscence is so violent that the 

 seeds are discharged with an explosive report. In Ecballium 

 the seeds are squirted out, together with some of the fruit 

 tissues, whence the name, squirting cucumber. In Impatiens 

 the fruit tissues are in a state of such deUcate balance that 

 a mere touch causes violent dehis- 

 cence and dispersal, whence the sig- 

 nificance of the scientific name as 

 well as of the common name, touch- 

 me-not. In a western mistletoe, 

 Arceuthobium occidentale, the ripe 

 fruits explode, ejecting the seeds for 

 several meters; as in other mistle- 

 toes, the seeds adhere readily to 

 leaves or bark. 



V — - 



1213 1214 



Figs. 1212-1214. — 1212, an opening pod or 

 legume of the lupine {Lupinus perennis), illus- 

 trating violent dehiscence through the torsion 

 of the valves iy) when desiccated; the seeds 

 (j) are mechanically expelled for some distance; 

 1213, 1214, dehiscence of the capsules of the 

 evening primrose (Oenothera biennis): 1213, a 

 mature capsule in which the four valves (v) are 

 beginning to split at the apex ; 1 2 14, a later stage 

 in which desiccation has caused the valves to 

 spread apart, exposing the seeds (s) in such a 

 way that they may readily be shaken out. 



In many cases there is no vio- 

 lent dehiscence, but the seeds lie 

 in such a position that a mechan- 

 ical impact causes scattering. 

 Most capsules (as in Oenothera, 

 figs. 1213, 1214, and Pedicidaris) 

 and many pods lie with their 

 valves open, and the wind may 

 shake out the seeds, or animals may brush them out. In many mints, 

 if one presses down a calyx having mature nutlets, the latter shoot 

 out upon release. Of especial interest is Polygonum virghiianum, 

 whose achene is fastened to an elastic cushion of tissue in such a way 

 that, when pressed back, it bounds off upon release for a distance of 

 three or four meters. Obviously the dispersal of seeds by propulsion 

 is relatively ineffective, since at best the seeds are scattered but a few 

 meters from the parent plant, and commonly much less. 



Dispersal by wind. — With seeds, as with spores, the most effective 

 of dispersal agents is the wind, especially from the standpoint of the 



