The Tropics Of South America

The Amazon basin, measuring over 2Vz million sq. mi., is for the most part a great plain, especially in the middle and lower reaches of the river. The South American forests, to which we turn because of an interest in Echinodorus species, do not have marshy characteristics but resemble temporary water reservoirs. It is true that the deep forests are moist, but they are entirely without permanent standing water. Where any great water surfaces are found, they are mainly concentrated in the vicinity of large rivers which during the rainy period overflow their banks and inundate the surrounding countryside. After the water has drained back into the main stream, a system of still backwaters and pools arises, often very extensive, which during the dry period gradually changes into marsh or dries out entirely.

There are two rainy periods in this area (January to April and November to December). Between these two periods there is a dry stage which is not so conspicuous as it is in the areas where there is only one rainy period and where, consequently, the dry period is longer.

Rainfall in the Amazon area reaches 20 to 25 meters a year, and it tends to rain more often in the upstream direction. As everywhere in the tropics, the rainfall is distributed unevenly so that during the rainy period the daily downpours cause floods. At this

This photo shows the typical marshy conditions of the natural habitat of Sagittaria montevidensis in the Amazon. Photo by H. Schulz.

time the water level of the Amazon rises by anything up to 15 meters (on the average, 3 to 6 meters) and the force of the flooding river destroys the vegetation. Only a few plants grow successfully year after year either in the river or on its immediate banks. In the vicinity of the river no trees grow and an open landscape arises, with the result that fluctuations of temperature during the day and over the course of the year "a re much greater than, for instance, in the jungles of southern Asia. In the main stream of the river only a very small number of annual species grow, and these are propagated by seeds or spores {examples are Victoria, the giant water-lily, Pistia, water lettuce, and Azolla, a fern).

Plant communities in the vicinity of the Amazon arise in accordance with the undulations of the terrain. In the lowest-lying places, which are flooded at any slight rise in the water level, no trees grow. During the rainy period the whole area is inundated

Note the scarcity of vegetation In this almost dry marshy terrain near Rio Araguala, Brazil. Photo by Dr. H.R. Axelrod.

and later (during the dry period) becomes muddy and quite impassable. Yet even here annual plants prevail to grow and during the short dry period produce seeds. Perennial plants do badly at this level and seldom survive to produce seeds.

From the point of view of the aquarist, the most interesting habitats are situated 6 to 9 meters above the normal level of the river and are flooded only during the highest waters of the rainy season. After the waters recede, extensive lagoons and marshes are formed as the ground gradually dries out. These provide suitable habitats for the amphibious species of Echinodorus. In the lagoons where the water remains all year, both amphibious and typically submersed plants are found, such as Echinodorus amazonicus, Cabomba, Heteranthera, Myriophyllum, and others.

The average annual temperature of the Amazon area is 77° to 79° F (25° to 26° C), but during the year the temperatures may vary from 57° F to as much as 104° F (14° to 40° C). The fluctuation in temperature is therefore considerable, explaining why the South American species of aquatic plants are generally very resis-

A river bank scene along the Rio Negro, a tributary of the Amazon River. Note the low vegetation In the background of the picture. Such vegetation is under water during the rainy season. Photo by Dr. H.R. Axelrod.

tant to temperature changes, the majority of them able to be cultivated successfully even in unheated indoor aquariums. The temperature of the river water in the Amazon basin varies from 65° to 86° F (18° to 30° C) in quiet areas of still water. It reaches an average of 83° to 86° F (28° to 30° C) in lagoons during the vegetative (dry) period, but during unusually warm periods shallow lagoons can reach as high as 104° F {40° C).

In the tropics of the Amazon we find variable habitat conditions that produce what is sometimes called a cycle of development in plants. During the rainy period there is a stage of inconspicuous vegetative rest caused by the mechanical effects of fast moving water and movement of soil driven by flood waters. In addition there is a lowered temperature and a lack of light because the muddy water transmits little sunlight. When the floods are over the submersed plants start to flower; with the development of leaves such plants as Echinodorus also sprout flower stalks reaching above the water surface or, as in Cabomba, develop both floating leaves and flowers. Amphibious plants develop leaves now but do not yet flower.

By the time the water level falls to 20 to 40 cm most submersed plants have finished flowering and the floral stalks of some types begin to take root after maturing, A period of vegetative propagation follows. On the contrary, amphibious species develop only floriferous stalks. This means that in one and the same place various kinds of plants grow in stages, one after the other, also flowering in sequence, a definite cycle of development.

Other changes in the composition of vegetation can be observed on the fringes of lagoons and still waters where the treeless formations of the savannas pass into marshes and on to the shallows of the river. Here the development of the plants is extraordinarily varied. The specific composition of the plant growth varies continually, the number of some species is increased, and others disappear temporarily or permanently. The plants very often change their shape and the size of their leaves (the so-called "leaf multiformity"). On the fringes of the waters, on the banks and in the shallows, are found such plants as Echinodorus quadricostatus, E. tenellus, and various species of Eleocharis. Toward the savanna zone are found various marsh plants of the Araceae family which are not capable of withstanding permanent inundation. Toward the lagoons, even in shallow water, plants of the genera Myrio-phyllum and Heteranthera grow, while everywhere on the surface of both shallow and deep still waters are many floating plants.

Imagenes Dragon Ball

This picture illustrates the variety and density of vegetation in the backwaters of Rio Negro, Brazil. Photo by Dr. H.R. Axelrod.

The aquatic and marsh plants of the Amazon area are thus characterized by a relatively great ability to withstand fluctuations of temperature. They tolerate ordinary indoor conditions and are not damaged by even'a fall of temperature to 54° or 59° F (12° or 15° C), The species are mainly amphibious and can be cultivated in both paludariums and aquariums. Distinctly submersed plants are very rare, and even these can grow emersed for a short time {Heteranthera, Cabomba). The South American plants of the genus Myriophyllum are better adapted to an emergent life than are species from all other continents except Australia.

The aquatic and marsh plants of tropical South America have no distinct resting period, growth ceasing only during those floods in which their leaves and more delicate stems are destroyed. That is why they can grow indoors all the year around and are content with a temperature of about 68° F (20° C). Illumination for about 12 hours a day suits them well at all seasons. Of the conditions in their environment, it is the depth of the water that mainly influences their development. Since they are characterized by a high resistance to external conditions, the South American plants are most suitable for the aquarium. In indoor aquariums we can prepare much better and more stable conditions than are offered in their natural habitat.

It should not be forgotten that a number of very decorative aquarium plants from South America do not grow in the tropical area proper, but in the subtropical and temperate zones of southern Brazil, Paraguay, Uruguay, and Argentina. Here are found the exquisite novelties of our aquariums, such as Echino-dotus osiris, E. horemanii, E, opacus, E. longiscapus, and many others. These species, growing permanently under water, are found in relatively cool conditions with temperatures about 58° to 67° F (14° to 19° C). This of course does not mean that they could not do well at the ordinary aquarium temperature of about 72° F (22° C).

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