COLONIZATION 327 



seeds enables them to slip throiigli other vegetation and 

 reach, soil in which they can germinate. If ripe capsules 

 of the Balsam {Impatiens noli-me-tangere) are subjected 

 to even the slightest touch they will immediately open, 

 the valves curling themselves up like watch-springs. 

 The sudden movement is accompanied by force safficient 

 to scatter the seeds to a great distance. There is little 

 wonder that this plant has been called " Touch-me-not." 

 The Wood-Sorrel {Oocalis acetosella) and the Cuckoo- 

 Mower {Cardamine pratensis) in dry weather scatter their 

 seeds in a forceful manner. The Squirting Cucumber 

 (Momordica elaterium), a native of Southern Europe, 

 produces a prickly green fruit which, when ripe, springs 

 from its stalk at the slightest touch. The seeds con- 

 tained in the fruit are embedded in a viscid fluid, and 

 when the fruit leaves the parent plant, they are expelled 

 with a whizzing sound, and with sufficient force to carry 

 them to a distance of several yards. In the examples 

 mentioned in this paragraph we have, then, instances of 

 seeds being scattered by expulsive force. 



In other cases water is a transporting agent. It must 

 be such in those aquatic plants that fruit under water; 

 but fruits and seeds of land-plants may be carried long 

 distances by streams and marine currents. The fruits 

 of the Cocoa-nut Palm {Cocos nucifera) are often carried 

 hundreds of miles by ocean currents, and washed ashore 

 while still able to germinate. The nut is enclosed in a 

 fibrous covering which makes the fruit buoyant. There 

 are air-spaces in Water-Lily fruits which enable them to 

 remain afloat for a time, and drift away from the parent 

 plant. In time the seeds are freed from their " floats " 

 by the rotting of the latter, and then they sink to the 



