Chapter VIII —127— Natural Factors 



In the plant as in the animal kingdom, there exists a natural ten- 

 dency toward multiphcation and the winning over of ever new terri- 

 tory for the progeny. To quote Darwin {I.e., p. 79), "Every organic 

 being naturally increases at so high a rate, that, if not destroyed, the 

 earth would soon be covered by the progeny of a single pair. . . . 

 Linnaeus has calculated that if an annual plant produced only two 

 seeds — and there is no plant so unproductive as this — and their 

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

 would be a million plants". Careful calculations made by Kerner 

 have shown that every year, on the average, one plant of Sisymbrium 

 Sophia produces 730,000 seeds, Nicotiana Tabacum — 360,000, Erigeron 

 canadense — 120,000, Capsella bursa- pasioris — 64,000, Plantago major — 

 14,000, Raphanus raphanistrum — 12,000, and Hyoscyamus niger — 

 10,000. Each of these seeds may give rise the next year to a new 

 plant, and each of these plants is capable of producing the indicated 

 number of seeds. 



In the lower plants the number of spores formed by one individual 

 plant is far greater. Ridley (1930, p. 64) gives the following data on 

 the number of spores produced by a single fungus: — 



Psalliota (Agaricus) campestris 1,800,000,000 



Coprinus comatus 5,000,000,000 



Polyponts squamosiis 11,000,000,000 



Lycoperdon Bomsla 7,000,000,000,000 



At such a rate of multiplication, even taking into account the small 

 percentage of individuals surviving, if natural factors in plant dis- 

 persal really had as much significance as many ascribe to them and 

 the plants dispersed in this fashion could readily become established in 

 the new localities and spread farther, the vegetation of the globe, 

 within the limits of the same chmatic zones, would be quite homo- 

 geneous. 



In addition to the foregoing arguments, we can cite a number of 

 other circumstances that compel us to be very cautious in evaluating 

 the significance of chance factors in the distribution of plants. First of 

 aU, it should be stressed that the various adaptations for the transport 

 of fruits and seeds very often do not facilitate the dispersal of plants as 

 much as might be expected. As we noted previously, Bentham has 

 shown that those species of Compositae having plumed fruits are not, 

 by any means, the most widely distributed species of this family. 

 Similarly, according to the same authority, such adaptations possessed 

 by the fruits of Compositae as hooked formations, spines, viscid exu- 

 dations, etc., to which are usually ascribed very great significance, on 

 close investigation prove not to play such an important role. Many of 

 the spiny Compositae, such as Acanthospermum, have a comparatively 

 limited area. Species of the genus Echinospermum (of the family 

 Boraginaceae) with prickly fruits are, as a rule, less widely distributed 

 than those of the genus Myosotis with smooth fruits. Similarly, the 

 burdock, Arctium lappa (of the Compositae), characterized by fruits 

 with armed bracts or burrs, has a considerably more restricted range 

 than Cirsium aroense. 



As another example, we may take Chermezon's investigation 



