a, Natural size; b, greatly enlarged; c, section, greatly enlarged.
On the sands and mud-flats a semi-desert exists, due in great measure to the shifting nature of the material and the difficulty which plants find in securing an anchorage in it. But in the upper parts, near high-water mark, a few land plants—notably the Glasswort (Salicornia europæa, [Fig. 2]), a fleshy little annual—colonize the dreary flats with tiny forests of dark green branches, and lower down many small Seaweeds flourish. Some of these, ramifying through the surface layers, help to bind together the shifting sand, and by entangling in their branches fresh particles, and by continued growth, tend to raise and consolidate the surface, to render it suitable for the immigration of land plants such as the Glasswort, and thus eventually to reclaim it from the sea.
Fig. 2.—Glasswort (Salicornia europæa).
a, Plant, 2/3; b, male flower; c, female flower, both enlarged.
It is in the depths of the ocean, however, that the greatest deserts of our globe are to be found. The luxuriant Seaweed gardens that decorate the shallower waters of the sea, especially where a rocky bottom provides secure foothold, dwindle rapidly as the depth increases, owing to the diminution of light, and when the coastal fringe is left they cease. In the inky darkness of the ocean depths, amid absolute stillness and a temperature little above freezing, plant life of any sort is unknown. Only the flinty skeletons of diatoms and other minute forms of vegetable life which inhabit the surface layers, raining slowly down throughout the ages, tell that plant life exists in the sea at all.
On land, the larger deserts are found in the coldest and in the hottest regions. Around the North and South Poles lie great areas where the perennial lowness of temperature and the consequent almost continuous covering of snow and ice render plant life impossible. But just as the Eskimo live under conditions which would be wellnigh prohibitive to inhabitants of more temperate regions, so many of the higher as well as the lower plants creep northward far beyond the Arctic Circle, where, awakening from a nine months’ winter sleep, they break from the still half-frozen ground to brighten the brief summer with their leaves and flowers and fruit. The flora of Greenland, for instance, which we generally think of as an ice-bound and inhospitable land, numbers some 400 species of Seed Plants. These live mostly on the cliffs and steep ground that fringe the coast, where they are clear of the great icefields which bury the interior of the country, and in many places descend as broad glaciers into the sea. But the life of these high northern plants is slow and difficult, as is evidenced by their paucity and their stunted stature. Later on we shall have to consider how they adapt themselves to the adverse conditions under which they exist ([Chapter VIII.]); and we shall find their life problems are reproduced in many respects by those of the interesting alpine plants which may be found nestling in the rock crevices of the higher mountains of our own country.
But the more familiar deserts of the world, those to which the mind turns when we use the term, are mainly due, not to absence of light as in the ocean depths, nor to want of heat as in the polar regions, but to failure of the water-supply. A vast desert region of this kind stretches across Northern Africa from west to east, and onward through Arabia, Southern Persia, and Baluchistan. Another, almost continuous with it, extends from the Caspian Sea across great plains into Central Asia, and on over vast mountain areas into Western China. Other similar deserts, familiar to us in word and picture, are situated in the south-western United States, Mexico, and South Africa. In all these tracts, with their diverse characters and diverse sparse floras, the scarcity of rain is the primary cause of their peculiar features. The dryness prevents a protecting covering of vegetation, and allows heat and cold—both sharply accentuated by the scarcity of the moderating influence of water in either soil or air—to pursue their work of disintegrating the surface, reducing the rocks to sand and dust, which the winds sweep hither and thither. In such circumstances plants exist under