296 BOTANY OF THE LIVING PLANT 



an uninhabited desert was exposed, lying at a distance of fully twelve miles from 

 the nearest vegetation. Since then a new Flora has sprung up upon the islands. 

 This has been studied at intervals. The late Dr. Treub, who visited Krakatau 

 in 1886, concluded that the first colonists were blue-green Algae associated 

 with Diatoms and Bacteria. These formed a suitable nidus for the spores of 

 Mosses and Ferns, and for the seeds of Flowering Plants adapted for dispersal 

 by winds. On the beach were found the fruits and seeds of Flowering Plants 

 carried by water, some of which had germinated ; many of them belonged to 

 the characteristic strand-flora of the Malay region. But the plants introduced 

 by animals or by man were not found by him on this visit, which took place 

 only three years after the eruption. In 1897 Penzig visited the island, and 

 estimated that of the Flowering Plants noted 60.39 p.c. had reached it by 

 ocean currents, 32.07 p.c. by wind agency, and only 7.54 p.c. had been trans- 

 ported by fruit-eating animals and man. On a subsequent visit by a party of 

 botanists in 1906, the results as stated by Ernst show that though these pro- 

 portions for Flowering Plants were not exactly maintained, still the largest 

 number were borne by water-transit, and the smallest by animal agency. Thus 

 for an oceanic island the most effective agency of transit is water ; wind- 

 carriage takes a middle place, and transit by animal agency is the least 

 effective of the three. These are the results which would naturally have been 

 anticipated. 



PLANT-POPULATION. 



A very important factor in the maintenance and spread of a species 

 is the actual number of germs produced. The third Chapter of 

 Darwin's Origin of Species deals with the geometrical ratio of increase 

 of living things. Following Malthus he there points out that there 

 is no exception to the rule that every organic being increases at 

 so high a rate that, if not destroyed, the earth would soon be covered 

 by the progeny of a single pair. Linnaeus had already calculated 

 that if an annual plant produced only two seeds, and their seedlings 

 next year produced two, and so on, then in twenty years there would 

 be a million plants. This is, however, a very slow rate of breeding. 

 The following table gives the results of careful computation by Kerner 

 of the number of seeds produced in a single season by an average 

 specimen of each : 



Henbane (Hyoscyamus niger) 10,000 



Radish (Raphanus Raphanistrum) - - 12,000 



Plantain (Plantago major) - - 14,000 



Shepherd's Purse (Capsella bursa-pastoris) - 64,000 



Fleabane (Erigeron Canadense) - 120,000 



Tobacco (Nicotiana Tabacum) ... 360,000 



Flixweed (Sisymbrium Sophia) - - 730,000 



