<5o PLANT GEOGRAPHY 



seeded Grasses, and "weeds," or "ruderal" (roadside) 

 plants, which are generally characterised by their profuse 

 seed -production. Annuals and biennials will spring up 

 first, but in the intermediate stage will often be mastered 

 by perennials. Wind-disseminated (anemochoric) trees, 

 such as Birch, Aspen, and Pine, will perhaps generally 

 precede berry -bearing, bird-disseminated species; while 

 shrubs will obviously be able to grow up sooner than 

 trees. Light-demanding trees will always precede shade- 

 enduring species. The longevity of trees, the fatal 

 effects of their shade .upon many low-growing species, 

 and the long periods during which their seeds in many 

 cases retain their power of germination, all tend to give 

 the tree-form the advantage in the ultimate stage of 

 the successions. 



THE ANIMAL ENVIRONMENT. The distribution of 

 plants is influenced by animals chiefly in two ways, viz. 

 the dependence of many species of plants upon insects, 

 and even upon special kinds of insects, for their pollina- 

 tion ; and the dispersal of spores and seeds by birds and 

 other groups of animals. 



While many herbaceous plants in all latitudes are 

 insect-pollinated, or entomophilous , it is noticeable that, 

 among trees, it is chiefly in the tropics and warmer 

 temperate zones that we have species with the gaily- 

 coloured, sweet-scented, honey -yielding flowers that are 

 indicative of this characteristic. Most of the trees of 

 colder latitudes are catkin-bearers or conifers, with the 

 precocious - flowering, pendulous, greenish blossoms, 

 extruded stamens, feathery stigmas, and abundant 

 pollen distinctive of wind-pollinated, or qnemophilous, 

 plants. It is also significant that in many small, wind- 

 swept, oceanic islands flying insects are few or absent, 

 and groups of plants elsewhere entomophilous are pol- 

 linated by wind. A striking instance is the cruciferous 

 Kerguelen Island Cabbage (Pringlea antiscorbutica R. 

 Br.) (Fig. i), which is confined to the island from which 

 it takes its name, the Prince Edward and the Macdonald 

 groups. 



The absolute dependence of Yucca filamentosa L. for 

 pollination upon one moth, Pronuba yuccasella, or of 

 Aconitum upon Bombus, is exceptional, while the similar 

 limitation in the case of the Vanilla orchid proved 

 a check to its cultivation in countries to which it 



