THE FACTORS OF DISTRIBUTION 45 



Tillandsia, on tropical forest-trees. The smallest known 

 seeds are those of some epiphytic orchids, which are so 

 light as to be readily supported even by the gentle 

 upward currents of air in the interior of a forest. The 

 set of powerful winds in one direction must be of 

 importance in the carriage of seeds from the mainland 

 to islands; and there is evidence that small and light 

 fruits and seeds, whether winged or plumed or neither, 

 have been carried upwards of twenty miles in one flight. 

 The minute spores of club-mosses and ferns are still 

 more readily carried in this manner. The flora of 

 oceanic islands, such as Ascension, consists largely of 

 ferns; and no less than eleven species were found on 

 the island of Krakatau three years after it had been 

 covered with lava. 





CHAPTER II 



THE SOIL 



THE nutritive medium in which plants grow is either the 

 bodies of other organisms, in the case of parasites, water, 

 or soil. The conditions of these nutritive media are 

 termed edaphic (from ZdaQos, a foundation); but the 

 science of soils is now often known as agrology (from 

 dyp6s, land). The physical condition, chemical com- 

 position, and living inhabitants of the soil are all of the 

 deepest significance with reference to plant-life and 

 distribution, though there is still some difference of 

 opinion as to the relative importance of the first two 

 classes of characters. 



THE ORIGIN OF SOILS. Soils are largely of inorganic 

 or mineral origin; but to some extent organic, the 

 former resulting from the disintegration of rocks. By 

 soil, in fact and in practice, we mean that portion of the 

 land -surf ace which is sufficiently disintegrated to be 

 penetrated by the roots of plants; and it graduates 

 from the generally looser surface soil, which commonly 

 contains some organic matter derived from the decom- 

 position of successive generations of plants and animals t 

 through the more compact and more exclusively inor- 

 ganic subsoil down to the solid rock. The disintegration 

 of rock, subsoil, and soil is largely the result of weathering. 



