190 MR. BUNBURY ON THE VEGETATION OF BUENOS AYRES 



tion also changes. The Rio Colorado, in S. lat. 40°, was observed by Mr. Darwin to form 

 a pretty accurate boundary -line between these two formations ; and he 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, which, according to the 

 accounts we possess, are so thinly scattered over the shingly plains of Patagonia, that the 

 aspect of the whole country is strikingly barren and miserable. That this change of soil 

 should be 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 relations of these soils to moisture. 

 The surface of Patagonia, composed of loose shingle, is singularly dry ; so much so, it is 

 said, that one may travel for many days together without meeting with a drop of water : 

 consequently, it is fitted for the growth of such plants only as can bear this remark- 

 able degree of drought ; and the character of the Patagonian Flora, as shown by all the 

 accounts, is just such as we should expect under these circumstances. On the other 

 hand, the clay and marl of the Pampas, and the soil, formed of decomposing granite, on 

 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 have no very definite boundary, but 

 to melt, as it were, into that of southern Brazil. About Porto Alegre, in Rio Grande do 

 Sul, in S. lat. 30°, and consequently little more than four degrees north of Buenos Ayres, 

 the botany has a thoroughly Brazilian character, notwithstanding the absence of great 

 forests. There are abundance of large and showy climbers of a tropical aspect, — species of 

 Bignonia, Echites, Malpighiacem, Sapindacece ; of arborescent Mimosece ; of shrubby Com- 

 posites, belonging to the same genera, Vemonia, Eupatorium and Baccharis, which abound 

 so much in tropical Brazil ; and a vast profusion of Myrtles. The numerous Perns of Rio 

 Grande are almost all common to that district and Rio de Janeiro, and among them are 

 two arborescent species, which contribute to give a tropical character to the Plora. Not a 

 few phsenogamous species, also, extend from the tropical parts of the South American 

 continent as far as Porto Alegre ; for example, — Inga semialata, Mutisia speciosa, Bac- 

 charis dracunculifolia, Gaylussacia imbricata, Echites longiflora, Pleroma virgattim, Mi- 

 crolicia alsinefolia, Eryngium Pristis, Eriocaulon caulescens ; besides others which range 

 still further south, to Monte Video, such as Baccharis trimera, Pterocaulon spicatum, 

 Achyroclme flaccida, BZydrocleis Humboldtii, and various grasses. 



On the other hand, the comparatively small number of Melastomacece, and the abun- 

 dance of herbaceous and half-shrubby Verbenece, in Rio Grande, indicate the approach to 

 the Argentine region. Some, indeed, of the characteristic species of Buenos Ayres, such 

 as Verbena erinoides and chamcedrifolia, range northwards as far as Porto Alegre. The 

 considerable degree of difference between the vegetation of this latter place and of the 

 northern shore of the Plata must, I conceive, be due to climate only, for there exists no 

 natural barrier, and, as far as I can learn, there is no difference in the geological consti- 

 tution of the country. I possess no precise information with respect to the climate of 



* See Darwin's Journal of Researches, 2nd edit. p. 75. 



