Chapter VIII 



NATURAL FACTORS IN THE GEOGRAPHICAL 

 DISTRIBUTION OF PLANTS 



In undertaking a study of the geographical distribution of plants, 

 one is, first of all, confronted by the question: Is the dispersal of a 

 species mass-like in character, the species as a whole gradually, step 

 by step, though perhaps unevenly, covering ever new territory until 

 some barrier puts a limit to its spread, or is its dispersal achieved by 

 sudden spurts, as a result of individual plants penetrating into distant 

 localities isolated from the main area? In the first case discontinuous 

 areas could result only from the dying out of the species in the inter- 

 vening territories or the isolation of the occupied regions due to 

 geological causes, and such areas would, consequently, have to be 

 regarded as relic. In the second case the cause of their discontinuity 

 would be the chance transport of plants or parts of plants, such as 

 seeds, a great distance from the mother plants. 



We know that plants possess various structural adaptations for the 

 dissemination of their spores, seeds, and fruits, and that the latter, by 

 means of these adaptations supplemented by various natural factors, 

 such as wind, sea and river currents, animals, birds, etc., may be 

 transported a considerable distance from their place of origin. In this 

 connection, however, another question arises: How far may seeds be 

 carried and still retain their germinating power? We have already 

 noted that there exists a limit in this respect, differing, of course, for 

 different species. In order to arrive at some conclusions and to give 

 an answer to the questions we have just put, we shall now take up the 

 natural factors which facilitate the dispersal of plants. 



Wind as a Factor in the Dispersal of Plants: — Atmospheric move- 

 ments induced by various air currents— especially the stronger move- 

 ments, winds — doubtless facilitate the dissemination of spores and 

 seeds for considerable distances from the place of their formation. The 

 lighter atmospheric currents are naturally of significance only for the 

 transfer of very light objects, which, by reason of their lightness, re- 

 main in the air, as if suspended, and move laterally and also vertically 

 at the sUghtest movement of the air. 



In the dispersal of cryptogams, their spores being exceptionally 

 Hght, even slight air currents may play a big role. Falck (1927) found 

 that the spores of Basidiomycetes and particularly of Ascomycetes, owing 

 to the turgor of the cell-sap at the time of maturation, are forcibly 

 ejected from the ascus, and that, even when the air is practically 

 motionless, these spores are borne upward due to atmospheric currents 

 induced by temperature conditions. Subsequently, winds and, in 

 general, the stronger horizontal movements of the air aid in carrying 

 them over considerable distances. 



Members of the staff of the U. S. Department of Agriculture 



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