878 LIMITS OF DISTRIBUTION. 



contrivances are brought into play, the aim of which is to accompHsh autogamy. 

 The open flowers of Viola sepincola are adapted to cross-pollination through the 

 agency of bees; should no cross take place, and no fruits be produced from the open 

 flowers which bloom above the ground, cleistogamous flowers, hidden under- 

 ground, develop and bring forth a number of fertile seeds as a result of the auto- 

 gamy which inevitably takes place within their closed floral envelopes. Viola 

 sepincola may also be taken as a type of those plants in which the fruits ripen 

 underground and produce seeds which germinate at the spot where they were 

 formed. Such plants have always been a source of wonder to botanists, and their 

 number is not large. The best-known examples are Arachis hypogcea, Cardamine 

 chenopodiifolia, Linaria Cynibalaria, Phryniv/m micans, Trifolium subterraneum, 

 and Vicia amphicarpa. If these plants were only to bring fruits to maturity 

 underground, or were to draw all their fruits below the gi'ound as soon as the 

 seeds were mature, in order that germination and the development of new plants 

 might ensue at that spot, their behaviour would imply a renunciation of dispersion 

 to any distance, and the phenomenon would be highly enigmatic. The puzzle is, 

 however, satisfactorily solved when we take into account the fact that all these 

 plants invariably have the chance of being dispersed to great distances either 

 before the fruits become concealed in the earth, or by means of a second form of 

 fruit which ripens above ground, and is evidently adapted to being scattered abroad 

 through the agency of animals, or by means of aerial or aqueous currents. 



LIMITS OF DISTRIBUTION. 



The results of careful computations of the numbers of seeds produced yeai-ly 

 by a few selected plants show that on an average a plant of Sisymbriv/m Sophia 

 yields 730,000, one of Nicotiana Tabacvm, 360,000, one of Erigeron Canadense 

 120,000, one of Capsella Bursa-pastoris 64,000, one of Plantago major 14,000, 

 one of Raphanus Raphanistrum 12,000, and one of Hyoscyamus niger 10,000. 

 Each of these seeds may give rise in the following year to a new plant, which, 

 in its turn, may produce a corresponding number of seeds. Accordingly, if a 

 Henbane-plant developed 10,000 seeds in one year, and 10,000 plants sprang from 

 those seeds next year, and themselves produced 10,000 seeds each, by the end of 

 five years ten thousand billions of Henbane-plants would have come into existence. 

 Now, as the entire area of the dry land on the earth is approximately one hundred 

 and thirty-six billion of square metres, and there is room for about 73 Henbane- 

 plants on one square metre, if all the seeds referred to in our hypothesis ripened, 

 the whole of the dry land would, at the end of five years, be covered with the 

 plants in question. In the case of Sisymiriwm Sophia, the normal multiplication, 

 if unchecked, would, in the course of three j^ears, cover an area 2000 times as great 

 as the surface of the dry land with plants. 



Any such exclusive occupation of the entire earth by one or a few species is 

 prevented by a variety of causes. As regards land-plants, the sea, separating one 



