THE FRUIT AND SEED-DISPERSAL 295 



Nettle, the Shepherd's Purse, and the greater Plantain arc tlie 

 commonest of these. 



All such devices for the dissemination of seeds as those described 

 are to be held as offsets to the limitation imposed upon plants by 

 their fixity of position. They are themselves immobile, as arc also 

 their seeds ; but a spread of their seeds is essential for the survival 

 of a species, or its spread to new stations. The measure of its success 

 may be illustrated by a few examples. Darwin remarks that no 

 cultivated plant has run wild on so enormous a scale as the Cardoon 

 Thistle {Cynara cardunculus) , introduced from Spain to La Plata. 

 It has spread so as to cover large areas to the exclusion of other 

 plants. Its " pappus " carries its fruits down the wind, after the 

 manner of other Composites, and its spread shows the effectiveness of 

 the method. A more recent example is that of Lantana acideata^ 

 a. native of Mexico, which was introduced into Ceylon as a garden 

 plant in 1828, and has since spread all over the island, taking up waste 

 land to the exclusion of other plants. It is spread by birds, which 

 eat its pulpy fruits. Where forest fires occur in Canada the " Fire- 

 Weed " [Epilohium angustifolhim) at once occupies the cleared 

 ground. It reaches the sterilised surface by its light seeds being 

 supported in the wind by superficial hairs. A census was made 

 some years ago of plants found growing in humus borne on the 

 stumps of pollard Willows near Cambridge. The seeds or fruits 

 from which they sprang would have had to be raised about eight 

 feet above the ground. Of the total of nearly 4000 records, 44.62 p.c. 

 were plants with fleshy fruits, 25.18 p.c. had winged or feathered 

 fruits or seeds, 16.47 P-C- had burred fruits, 10.75 p.c. had seeds 

 so light or small as to be easily wind-borne. Thus the presence of 

 all the plants observed upon the stumps, excepting about 3 p.c, is 

 accounted for by recognised methods of seed-dispersal. Such 

 examples show the practical results of transport of seeds by the 

 methods described, and give some idea also of the rapidity of their 

 effects. 



An experiment on the grand scale was made in the formation of a coniplelel\- 

 new Flora of the Island of Krakatau, and fortunately its results were followed 

 by competent observers, who kept careful records. Tliesc form the beat 

 authenticated story of the natural formation of a plant-population in an area 

 where none was Hving before. Up to 1883 the islands forming the small group 

 in the Sunda Strait between Java and Sumatra, of which Krakatau is the 

 largest, were covered by dense vegetation. From May to August of thai 

 year successive volcanic eruptions resulted in the complete stcrilisaUon of 

 the surface, which was covered with hot stones and ashes. Thus, on coohng 



