AGGREGATION AND MIGRATION 247 



it plays an important part in the rock fields and gravel slides of 

 mountains, especially in the case of immobile species. 



6. Glaciers (crystallochores). Transport by glaciers is of slight 

 importance at the present time, because of its restriction to alpine 

 and polar regions, where the flora is poorly developed. In con- 

 sidering the migrations of the glacial epoch, however, distribution 

 by glaciers is an important factor. 



7. Growth (blastochores). The mol^ility of species dissemi- 

 nated by the growth of offshoots is extremely slight, and the annual 

 movement relatively insignificant. The certainty of migration 

 and of ecesis is so great, however, and the presence of offshoots 

 so frequent in terrestrial plants that growth plays an important 

 part in migration, especially within formations. 



8. Propulsion (bolochores) . Dissemination by mechanical pro- 

 pulsion, though it operates through insignificant distances, is very 

 important on account of its cumulative action from year to year. 

 The number of plants with contrivances for propulsion is very 

 much smaller than the number of those with offshoots. All species 

 of this group agree in having modifications by which a tension is 

 established. At maturity this tension suddenly overcomes the re- 

 sistance 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 are 

 classified as follows: 



(a) Hygroscopic fruits. These include the ferns with annulate 

 sporangia in which the expansion of the annulus by the absorjition 

 of moisture bursts the sporangium more or less suddenly. The 

 actual propulsion of the spores seems to be caused by the reflex 

 movement due to drying. 



(6) Turgescent fruits. Propulsion by turgescence occurs in a 

 large number of fungi, such as the fleshy Discomycetes, etc. Among 

 flowering plants, Impatiens and Oxalis are familiar examples of 

 fruits which split in consequence of increased turgidity. 



(c) Dry fruits. The number of fruits which dehisce ujion 

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

 seeds forcibly. Erysimum, Lotus, Viola, and Geranium illustrate 

 the different ways in which drying brings about the sutlden splitting 



of fruits. 



(d) Mortar fruits. In some plants, especially composites, 

 boraees and mints, the achenes or luitlets are so pln.cl in tlie 



