46 INVASIOU 



by falling or more frequently, by the breaking away and 

 rolling down of rock or soil masses and particles. Dissem- 

 ination by this method is relatively insignificant, though it 

 plays an important part in the rock fields and gravel slides 

 of mountain regions, particularly in the case of immobile 

 species. 



6. Glaciers, crystallochores (GrystallocTioras, Kpuo-roXXos, o, 

 clear ice). At the present time, transport by glaciers is of 

 slight importance, because of the restriction of the latter to 

 alpine and polar regions, where the flora is poorly developed. 

 In the consideration of migrations during the glacial epoch, 

 however, it plays an important point. 



7. Growth, blastochores {Blastochorae, fiXda-Tri, ■q, growth). 

 The mobility of species disseminated by offshoots is ex- 

 tremely slight, and the annual movement relatively insigni- 

 cant. The certainty of migration and of ecesis, is, however, 

 so great, and the presence of offshoots so generally the rule 

 in terrestrial plants that growth plays an important part in 

 migration, especially within formations. 



8. Propulsion, bolochores {Bolochorae, /SoXj;, ^, a throw). 

 Like growth, dissemination by mechanical propulsion, 

 though operating through insignificant distances, exerts an 

 important effect in consequence of its cumulative action. 

 The number of plants, however, with contrivances for pro- 

 pulsion is very much smaller than the number of blasto- 

 chores. All bolochorous species agree in having modifi- 

 cations by means of which a tension is established. At 

 maturity, this tension suddenly overcomes the resistance of 

 sporangium or fruit, and throws the enclosed spores or seeds 

 to some distance from the parent plant. In accordance with 

 the manner in which the tension is produced, sling-fruits 

 may be classified as follows. 



(a) Hygroscopicity, plado boles {Pladobolae, irXa8os,o, mois- 

 ture, -/SoXos, thrown). These include the ferns with annulate 

 sporangia, in which the expansion of the annulus by the 

 absorption of moisture, bursts the sporangium more or less 

 suddenly, though the actual propulsion of the spores seems 

 to come later as a result of dessication. 



