466 Linnaan Society. 



however did not exist in these countries until introduced by the 

 indirect agency of man. 



; The social character, so eminently conspicuous in many of the 

 .Pftturalized plants, is observable also, though in a less degree, in 

 several of the indigenous plants of the Pampas of Buenos Ayres, the 

 most remarkable cases observed being Verbena erinoides, V. chanue- 

 dryfolia, Mitracurpum Sellovianum, and a dwarf Solanum, besides a 

 few grasses. This social growth of some particular plants, and the 

 consequent uniformity of vegetation, has, Mr. Bunbury thinks, been 

 before noticed as characteristic of extensive plains. Tropical forms 

 of vegetation occur chiefly on the banks and islands of the principal 

 rivers. They are principally woody climbers, such as Passiflora 

 carulea, Stigtnaphyllum littorale, two or three species of Paullinia, a 

 Cardiospermum and a Bignonia ; or Leguminosce of a tropical cha- 

 racter, species of Mimosa, Inga, Calliandra and Cassia. One solitary 

 species of Melastomacece (an Arthrostemma) reaches to the north 

 bank of the Plata, but does not cross it. One Mach<Brium grows in 

 the islands of the Uruguay near its mouth. A few monocotyledo- 

 nous genera, which have their head- quarters within the tropics, ap- 

 pear for the last time on the banks of the Plata ; such are Canna, 

 Oncidium and Tillandsia. Palms, it would appear from Mr. DarM'in's 

 statements, occur here and there as far as 35° S. lat., which seems 

 to be likewise their southern limit in Chile. 



The southern limit of the Argentine vegetation seems to be deter- 

 mined mainly by soil, the northern by climate alone. Where the 

 calcareous mud and marl of the Pampas are succeeded by the arid 

 gravel or shingle of Patagonia, that is to say about the Rio Colo- 

 rado, in 40° S. lat., Mr. Darwin notices the change in the vegetable 

 covering of the soil, accompanying this change in its mineral nature. 

 The herbaceous vegetation, which clothes the surface of the Pampas 

 pretty uniformly, is succeeded by low, scraggy, thorny shrubs and 

 dry meagre grasses, so thinly scattered over the shingly plains of 

 Patagonia, that their aspect is strikingly barren and miserable. 

 That this change of soil should he attended with so great a change 

 in the vegetation, while that (more striking in a geological view) 

 which takes place when we cross the Plata seems to have very little 

 influence on it, is easily accounted for by the different relation of 

 these soils to moisture. The loose shingly soil of Patagonia is so 

 singularly dry, as to be fitted only for those plants which can bear 

 an extraordinary degree of drought ; while the clay and marl of the 

 Pampas, and the decomposing granite of the north side of the Plata, 

 are both sufficiently favourable to the retention of moisture, and 

 consequently to the growth of an abundant herbage. 



To the northward the Argentine region appears to melt as it were 

 insensibly into that of Southern Brasil. About Porto Alegre, in 

 S. lat. 30°, and consequently little more than 4° north of Buenos 

 Ayres, the botany has a thorough Brasilian character, notwith- 

 standing the absence of great forests. The numerous ferns of Rio 

 Grande are almost all common to that district and to Rio de Janeiro. 

 Not a few tropical jihsenogamous species also extend as far as Porto 



