INVASION 217 



5. Gravity, clitochores. The members of this group are exclusively col- 

 line, montane, and alpine plants, growing on rocks, cliffs, and gravel-slides 

 (talus), etc., in which the seeds reach lower positions merely by falling, or 

 more frequently by the breaking away and rolling down of rock or soil 

 masses and particles. Dissemination by this method is relatively insignifi- 

 cant, 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. 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. The mobility of species disseminated by off- 

 shoots is extremely slight, and the annual movement relatively insignificant. 

 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. Like growth, dissemination by mechanical 

 propulsion, though operating through insignificant distances, exerts an im- 

 portant effect^ in consequence of its cumulative action. The number of 

 plants, however, with contrivances for propulsion is very much smaller than 

 the number of blastochores. 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, pladoboles. These include the ferns with annulate 

 sporangia, in which the expansion of the annulus by the absorption of mois- 

 ture bursts the sporangium more or less suddenly, though the actual pro- 

 pulsion of the spores seems to come later as a result of dessication. 



(b) Turgescence, edoboles. Dissemination by turgescence is highly 

 developed in Pilohohis and in Discomycctes, though in the latter turgescence 

 results rather in placing the spores in a position to be readily carried by 

 the wind. Impatiens and Oxalis furnish familiar examples of fruits which 

 dehisce in consequence of increased turgidity. 



(c) Dessication, xerioboles. The number of fruits which dehisce upon 

 drying is very large, but only a small portion of these expel their seeds 

 forcibly. Geranium, Viola, Erysimum, and Lotus illustrate the different 

 ways in which dessication effects the sudden splitting of fruits. 



{d) Resilience, tonoboles. In some plants, especially composites, labiates, 

 and borages, the achenes or nutlets are so placed in the persistent calyx or 



