4] DISPERSAL AND MIGRATION 121 



travel to almost all parts of the world in a matter of days, and 

 traverse vast distances in a very few hours. 



Mechanical Dispersal 



Although it is usually effective over only short distances, mech- 

 anical propulsion or even extensive growth can be of distinct 

 advantage in migration. Thus plants which shoot out their dis- 

 seminules can thereby launch them into a goodly wind or on to a 

 passing animal that will carry them for miles. And often it is 

 agitation by wind or an animal which sets off the explosive mech- 

 anism. Furthermore, the aggressive growth of overground runners 

 (Fig. 28, B) and underground stems (rhizomes, see Fig. 28, A) often 

 give plants a distinct advantage in competition over their neigh- 

 bours, so that when, as is often the case, the peripheral growth is 

 detached as a separate plant, for example by the death of the parent, 

 it may be established at an appreciable distance ; and such distances 

 mount up usefully through the generations. As examples. Ground- 

 ivy {Glechoma hederacea) can trail a distance of 20 feet (about 

 6 metres) along the ground, and Elms can reproduce by suckers from 

 underground roots at fully 50 yards (about 46 metres) from the 

 parent tree. Even such growth as that of the Walking Fern shown in 

 Fig. 22 leads, in due course, to a worthwhile amount of dispersal. 



Particularly effective are the explosive spore-discharging mech- 

 anisms of some Fungi, which, usually on sudden rupture to relieve 

 stresses, may shoot their spores or spore-producing organs in some 

 instances as much as 15 feet. However, in the case of spores, a 

 tiny distance to take them into the free air is often sufficient to 

 launch them in atmospheric currents that may carry them practically 

 anywhere. Also capable of being shot out for distances of as much 

 as 3 feet are the bulbils of some Club-mosses [Lycopodium spp.). 

 Better known, however, are the explosive mechanisms of some fruits, 

 of which examples are shown in Fig. 28 and more may be cited. 

 The ' records ' seem to be held bv species of a genus of small 

 parasitic Mistletoes {Arcetithobium), followed by tropical American 

 trees of the Spurge family (Euphorbiaceae), particularly the Sand-box 

 Tree {Hura crepitans), which can throw its seeds more than forty 

 feet, and Para Rubber {Hevea brasiliensis), whose performance is 

 nearly as good. The explosion of a Hura fruit is spoken of as a 

 ' regular detonation '. In a similar manner, on drying of the fruits, 

 even small herbaceous members of the Spurge family often shoot 

 out their seeds for a distance of a dozen or more feet. In the case of 



