The botanical characteristic of the Pampean prairie is the absolute lack of trees. The study of the plant associations that cover it is not yet completed; its dominant primitive vegetation is constituted by the Graminaceae of the genus Stipa, commonly called flechillas for their flowers equipped with long restas with fine hairs when the fruits ripen; this gives the plain the appearance of an endless carpet. Other genera of Graminaceae represented in it are: Aristida, Andropogon, Chloris, Poa, Bromus, Paspalum, Setaria, Festuca, Agrostis, Eragrostis, Melica etc.
River waters are deposited in the dead-end depressions of the land and permanent or temporary lagoons are created. The flora in these basins is not very varied and is characterized by the presence of juncos (Scirpus riparius). In other cases the totora (Typha dominguensis Pers.) Dominates or the cortadera (Cortadera dioica Spreng. Speg.) Emerges with its white plumes, a plant that gives a singular note to the complex. Together with the Giraminaceae mentioned above, Composite, Leguminosae, Solanaceae, Geraniacee, Oxalidacee, (vinagrillos, very abundant), Verbenacee (the margarita punzó with beautiful flowers), Umbelliferae (Eryngium, from the appearance of Bromeliads), Amarillidacee (Zephyranthes and Hippeastrum), Iridaceae (Sisyrinchium), etc.
In the south of the province of Buenos Aires there are two systems of sierras of archaic character: those of the Ventana and of the Tandil, with a very rich flora especially on the NE side, because it is protected from the always cold and rough winds of the southern quadrants. The fissures of these stony masses, whose maximum height is 1000 meters in certain peaks, are covered by ferns, mosses and lichens and there are other plants such as amor seco (Blumenboachia, Loasacea), the very curious Plantago Bismackkii, commonly called llantén, semi- woody branched with tufted, gray, linear leaves; and many other species of different families.
In these mountains there are woody vegetables, such as the Ramnacea spinosa called brusquilla (Discarea longispina) whose very developed roots contain abundant tannin; this plant is not exclusively serrana, but also descends to the plain, in hard soils. We will remember for this characteristic flora the Pernettya mucronata, Ericacea patagonica and the curamamoel (Colletia cuciata) another Ramnacea aphilla, with very thorny, opposite, alternate branches in the shape of scalene triangles with the minor cathetus facing up, fiercely pungent.
These sierras (de la Ventana and Curá Malal) have been botanically studied by doctors Holmberg and Spegazzini, who have recognized 72 families and 700 species in their flora.
It is necessary to point out the increase that exotic species have taken in this formation, in the most cultivated and populated region; many of these species are favorable to pastoralism, such as certain Graminaceae (Briza minor, Bromus mollis, Poa spp., etc.), some Leguminosae and Geraniacee, etc., while other introduced species are harmful for agriculture such as cardos (Cynara, Carduus and other genera), abrepuño (Centaurea), abrojo chico (Xanthium spiñosum), the abrojo grande which is widespread and is an officially recognized plague; it is a species native to America and not to Europe as was believed until recently and its scientific name is Xanthium Cavanillesii.
The plow and the presence of sheep favor the development and diffusion of this exotic vegetation; but if cattle are allowed to graze in a field where these plants abound, the primitive vegetation formed by strong pastures soon returns.
Lombú (Phytolacca dioica L.) which is erroneously is considered as an indigenous plant of Pampa prairie, is an herb that lives spontaneously in small woods that grow on the banks of the Plata and Parana, associated with tala (Celtis), coronillo(Scutia), sombra del toro (Iodina), and in the provinces of Santa Fe, Entre Ríos and Corrientes; the latter province seems to be the center of dispersion. This vegetable was planted in the Pampa near the houses and as a signal from the first settlers to orient themselves in the immensity of that expanse of land whose monotony was absolute. It should be considered as a grass, because its trunk is fleshy and juicy, and a three-year-old branch, for example, can have up to 20 circles; which is not found in trees proper.
According to picktrue.com, the south of the province of Corrientes and the whole province of Entre Ríos make up the Mesopotamian formation.
In large part it is formed by the vegetable association called Selva di Montiel which reaches a few leagues from the great rivers Paraná and Uruguay. It is a very homogeneous, open type of wood, in which Leguminosae-Mimosoidaceae predominate: espinillo (Acacia cavenia), algarrobo (Prosopis), nandubay (Prosopis), quebracho blanco (Aspidosperma), and also has the characteristic of presenting other elements quite constants; the Trithrinax campestris palm(Brum.) Drude et Gris., Which we have already mentioned for the formation of the wood, which occurs in groups of 6 to 8 individuals, each of which germinates from the base of the other. Another type of palm, less abundant and of which there are colonies near the city of Concordia (Entre Ríos), is the yatay (Cocos yatay Mart.).
Montiel’s terrain is highly undulating, has streams, marshes and lagoons. The hillsides are rich in highly developed pastures. The broad and slightly bumpy peaks are more sterile, the vegetation is lower and the wood often presents itself under the almost uniform appearance of shrubs. Together with the previously named elements, they are found in these spots: chañar (Gourliea), coronillo (Scutia), ceibo (Erythrina), molle (Schinus), and other sub-herbaceous and herbaceous species including the Cactaceae. Stony outcrops do not exist in Montiel, except the rocks of the lowering of the rivers, whose beds are formed by a clayey earth, sometimes mixed with small stones. Along the Paraná the appearance of the banks changes, with the appearance of tertiary formations.
The uniformity of Montiel is only interrupted by the open fields and by the small hygrophilous woods of certain rivers and streams. The former are largely due to the deforestation carried out for the exploitation of wood, with which the coal that is consumed in Buenos Aires is made. These open fields are intended for agriculture. They preferably grow wheat, maize and oats. The small woods along the banks of the waterways have an exuberant vegetation that contrasts with the previous one. There is a preponderance of Mirtaceae and Euforbiacee, in addition to the vivaro (Ruprechtia).
Different from the uniformity of Montiel is the vegetation of the Paraná bank which forms a relatively narrow band, limited in some cases to the banks of the river. In the favorite places the flora shows itself in all its splendor and is different for some elements from that of the Uruguay river and its tributaries. Essences common to the two types are: Inga, Erythrina, Eugenia, Scutia, Rapanea, etc., but while there are no plants found on the slopes of Uruguay, there are others that represent the flora of that region in the wood of Córdoba and in the Chaco, such as mistol (Zizyphus), guayacán (Porlieria) , The Timbó (Enterolobium), etc.
This is, briefly, the appearance of the Argentine vegetation, originating from the derivations of three fundamental centers: the tropical flora of the Andes in the West, that of the Brazilian massif in the E., and, in the southern regions, a special type of vegetation, undoubtedly linked to areas of the continent that have now disappeared.