DISPERSAL AND MIGRATION 



123 



Fig. zH. — Dispersal by extension of growth or by mechanical propulsion, etc. 

 A, horizontal rhizome of a Grass (the sand-binding Marram Grass, Awmophila 

 arenaria ( ■; about \)\ B, Strawberry (Frogario) runner establishing daughter 

 plant ( < I); C, fruit of a Balsam {hiipatiens bolsamina), which explodes and 

 scatters the seeds (x 1); D, seed dispersal in the Squirting Cucumber {Ecballium 

 eloteriiim ( ■ about 5); E, ripe fruit of Pansy {Viola sp.), showing seeds ready 

 to be, and being, shot out (x 2); F, over-ripe fruit of Geranium {Geranium 

 sp.), showing slings which have thrown out the seeds ( X about 2^). 



one species of Arceuthobiiim the tiny bullet-shaped seeds have been 

 reported to travel over 66 feet (about 20 metres) from a point 8 feet 

 above the ground, and in one instance large numbers were collected 

 from the roof of a cabin one-quarter of a mile (about 402 metres) 

 from the point of liberation — presumably after transportation by 

 wind, though the seeds are also viscous and apt to be carried by birds. 

 Such a combination of explosion and adhesion is utilized also by 

 the Squirting Cucumber {Ecballium elaterium), shown in Fig. 28, D. 

 When the fruit is ripe, it breaks from the stalk, and through the hole 

 thus left the internal pressure is relieved by the seeds being ejected 

 with an abundance of mucilage and with such force that they com- 

 monly fly for several feet through the air. A slight touch will send 

 off the ripe fruit, so that a passing animal is liable to receive a broad- 

 side — and, incidentally, carry the adhering seeds much farther. 

 There are many varieties of this type of turgor-engendered explosion, 

 another being exhibited by certain Cresses {Cardamine spp.) that 

 have explosive pods of which the narrow valves, on being touched 

 when ripe, suddenly curl outwards with some violence, shooting out 

 the seeds — sometimes for more than 2 feet. In the case of such 

 diminutive plants, this is ample to take the seeds away from the 

 parental sphere of influence. Notable among the rather many and 

 diverse plants which do this sort of thing are the Balsams [Impatiens 

 spp.), in which the wall of the fruit is made of three layers of which 



