The Sesuvium Zone (The Peruvian Coast).
This zone, which comprises the whole Peruvian sea-border from Arica in 18°30ʹ S. to the vicinity of Tumbez in about 3°30ʹ S., usually possesses in its scanty littoral flora one or two species of Sesuvium, and in some places Sesuvium alone occurs on the beach. The beaches here do not line a region of almost complete aridity, as in the coast corresponding to the great desert region of North Chile. Though here also scarcely any rain falls, the sea-border receives the benefit of the “garuas” or drizzling sea-fogs; whilst the region immediately behind the coast may either be desert or semi-sterile during much of the year, or may be scantily vegetated, or, as along the river-valleys, may display a vegetation more fitting to the latitude. The general aspect, however, of the coast of Peru is one of aridity; but there are probably few beaches where a certain amount of subsoil drainage from the land sea-ward does not exist. This is well exhibited at Ancon, north of Callao, where in the most unlikely situations water is reached by digging wells; but in spite of this the Sesuvium alone grows on the beach. The beaches examined by me in the heat of the day in February, as at Mollendo and Ancon, had much the same surface-temperature noticed in the preceding month on the beaches of North Chile, namely, 120° to 130° F., and in one place 135°; whilst at a depth of four inches the sand was rather cooler, and instead of being between 95° and 100°, as on the Antofagasta and Iquique beaches, it was here usually only about 90°. But it was only occasionally that the sand felt at all moist at a depth of five or six inches; and in this zone, therefore, only a few shore plants of a peculiar type could be expected to find a station on the beaches, excepting, of course, those localities where low marshy districts or lagoons lie behind the beach.
The beach plants of the coast of Peru as observed by me though usually scanty, presented two types according to the character of the district bordering the beach. I make no mention here of those local plants, often belonging to the Compositæ, that as at Callao and Arica descend the valleys to the beaches, or to those numerous introduced plants that accompany cultivation, such as we find at Arica. In those coast localities, as at Arica, Callao, and Chancay, where salt-water pools or brackish lagoons lie behind the beach, or where a stream or a river empties into the sea, Sesuvium portulacastrum, Heliotropium curassavicum, and a Salicornia are to be generally noticed, and, as at Callao, Batis maritima may also abound. On the Chancay coast, about 30 miles north of Callao, there lies inside the shingle-beach a large shallow lagoon of brackish water (spec. gr. 1·012) with extensive muddy marginal flats, the temperature of the water at the edge being at mid-day on Feb. 3rd, 90° F. In the water flourished Ruppia maritima, which was also exposed in dead, dry, matted masses on the bordering mud-flats. On these mud-flats grew Sesuvium portulacastrum, which near the water’s edge was associated with a small species of Salicornia, whilst further away from the water it was accompanied by Heliotropium curassavicum.
But the most typical beach-flora of the Peruvian coast is such as we find on the dry beaches skirting the base of sand-covered or barren hill-slopes such as occur at Mollendo, Ancon, and Paita. As at Ancon, sand-covered hills and plains may extend miles inland, displaying here and there lines of shifting sand-mounds or “medanos.” On such beaches we may often find only a solitary plant, a species of Sesuvium which seems to differ only in its larger flowers, its much larger leaves (2 inches long), and its stout stems, of the thickness of the little finger, from the ordinary Sesuvium portulacastrum. This seems to be the only plant that can make its home on such beaches. At Mollendo, where there are signs of desiccated pools behind the beach which are occasionally filled with sea-water, the vegetation was of an intermediate character and more abundant; and here grew Sesuvium portulacastrum, a tall Salicornia, and Suæda fruticosa; whilst the commonest plant was a prostrate Nolanaceous species with a handsome purplish flower.
Excepting with the fruits of Batis maritima, and perhaps the buoyant joints of Salicornia, scarcely any of the prevailing shore-plants of the coast of Peru possess a capacity for dispersal by currents. In this zone I rarely found any seed-drift on the beaches. Much rubbish, such as roots of bamboos, however, may be brought down by the rivers; and where the Humboldt Current strikes a bend in the coast we get a repetition, on a smaller scale, of the scenes on the Antofagasta beaches. Ancon Bay, for instance, receives much of the floating offal of the south.