342 JOUKNAL OF THE KOYAL HOETICULTUEAL SOCIETY. 



REMABKABLE INSTANCES OF PLANT DISPERSION. 

 By Rev. Professor G. Henslow, M.A., V.M.H. 



[Eead October 26, 1909.] 



It is a well-known fact that some plants are very widely dispersed, not 

 only in a particular and more or less circumscribed locality, but sometimes 

 over a continent, or, indeed, over the whole world, except where extremes 

 of climate forbid their presence. 



The question arises, What are the peculiarities of such "cosmo- 

 politan " plants that enable them to thrive and multiply enormously in 

 a vast number of places ? 



There are two methods of multiplication : (1) the vegetative, by 

 means of the roots, stems, bulbs, &c. ; and (2) the sexual, or by fruits 

 and seeds. 



By the former method, in the case of runners and suckers, the area 

 colonized by an individual plant is comparatively limited. If it be a 

 water plant, and if the stem can break up into joints, then the stream 

 will carry them along, so that they may strike root at various places. 

 By such means Elodea canadensis has become widely dispersed through 

 the British Isles since 1836, when it was presumably first and accidentally 

 introduced with timber. Only the female plant is here known, except in 

 one or two spots. 



The best illustration known to me of great multiplication and wide 

 dispersion by bulbs is to be seen in the northern distribution of Oxalis 

 cernua, Thunb.* This plant is a native of the Cape of Good Hope, and 

 has often been described by botanists who have studied the Cape flora.t 

 It has become distributed not only on the islands of the Atlantic, as the 

 Bermudas, Canaries, and Madeira, but along both the north and south 

 coasts of the Mediterranean Sea. At the present day it is found in 

 various places, from Egypt to Morocco, and from Gibraltar to the Greek 

 islands. How did it come to be so widely distributed throughout the 

 Mediterranean region ? 



No botanical author previous to the last century ever refers to it as 

 growing in the northern hemisphere. The first to allude to it is Father 

 F. C. Giacinto, who mentions the plant as being cultivated in the Botanic 

 Garden at Valletta, Malta, in the year 1806. The title of his work is 

 " Index Plantarum Horti Botanici Melitensis, anno 1806.— P. F. Carolus 

 Hyacinthus." i 



* Thunberg, Diss. No. 12, t. 2, f. 2 (1794). 



f E.g. Harvey and Sonder, Flora Capensis, i. 348; and Thunb. Fl. Cap. ed. 

 Rchult. 537 (1825). Schultes describes Oxalis cernua as flowering at the Cape in 

 June and July. It lias completely changed this habit in the north, as it is in full 

 blossom all through the winter, from November to April. 



X A copy of this work is in the public library at Valletta ; and I take this oppor- 

 tunity of thanking Dr. A. Caruana Gatto for kindly making researches for me among 

 the publications therein contained. 



