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 aceurate 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 elay 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, Malpighiacee, Sapindacee ; of arborescent Mimosee ; of shrubby Com- 
posite, belonging to the same genera, Vernonia, Eupatorium and Baccharis, which abound 
so much in tropical Brazil; and a vast profusion of Myrtles. The numerous Ferns 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 Flora. Nota 
few phænogamous 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 virgatum, Mi- 
crolicia alsinefolia, Eryngium Pristis, Eriocaulon caulescens; besides others which range 
still further south, to Monte Video, such as Baccharis trimera, Pterocaulon spicatum, 
Achyrocline flaccida, Hydrocleis Humboldtii, and various grasses. ship 
On the other hand, the comparatively small number of Melastomacee, and the abun- 
FER * oe aii mule Ferbenen, in Rio Grande, indicate the approach to 
Ut akinpe g1 ce " ur. of the characteristic species of Buenos Ayres, such 
considerable degree of difference seras ne "enin ^ a en Fatte Alogi «dili 
TL she een the — of this latter place and of the 
natural barrier, and, as far as I ka i = : Saad ups pea papa = 
tution of the icai bs er — wie Rb leet oisit 
"UM. ^ possess no precise information with respect to the climate of 
* See Darwin’s Journal of Researches, 2nd edit. p. 75. 
