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. AJl 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 Pilobolus and in Discomycetes, though in the latter turgescence 
results rather in placing the spores in a position to be readily carried by 
the wind. Impatiens and Ovalis 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 
