difficult to regard “coniferous forests” as a natural ecological
group. At much higher altitudes, in the south-west of the Mediterranean
region, forests occur of the Atlantic cedar (Cedrus atlantica).
These occur from about 4000 ft. (1219 m.) to about 7000 ft. (2133 m.)
on the Atlas Mountains. Some sclerophyllous forests of the eastern
Atlas Mountains are, owing to a comparatively high rainfall,
characterized by many deciduous trees, such as Fraxinus oxyphylla,
Ulmus campestris (auct. alg.), Alnus rotundifolia, Salix
pedicellata, Prunus avium, &c.; and thus they have some elements
in common with the deciduous forests of central Europe.
The forests of these subtropical and warm temperate regions are situated near the sea or in mountainous regions, and (as already stated) are characterized by winter rains. In inland localities, where the rainfall is much lower, steppes occur. For example, in southern Algeria, a region of steppes is situated on a flat plateau, about 3000 ft (914 metres) high, between the southern slopes of the Tell Atlas and the northern slopes of the Saharan Atlas. The rainfall, which occurs chiefly in winter, only averages about 10 in. (254 mm) per annum. Here we find open plant associations of Halfa or Esparto Grass (Stipa tenacissima) alternating with steppes of Chih (Artemisia herba-alba); and each plant association extends for several scores of miles. In the hollows of this steppe region, salt water lakes occur, known as Chotts; and on the saline soils surrounding the Chotts, a salt marsh formation occurs, with species of Salicornia, some of which are undershrubs.
Where the rainfall is still lower, deserts occur. At Ghardaia, in south-eastern Algeria, the mean annual rainfall, from 1887 to 1892, was about 4½ in. (114 mm.). In 1890, it fell as low as 2 in. (53 mm.) (Schimper, 1903: 606). At Beni Ounif and Colomb Béchar, in south-western Algeria, I was informed, in March 1910, that there had been no rain for about three years. Here the gravelly desert is characterized by “cushion plants,” such as Anabasis aretioides; by “switch plants,” such as Retama Retam; and specially by spiny pants, such as Zizyphus Lotus and Zilla macropteris; whereas succulent plants are rare. Both in the steppe and in the desert, small ephemeral species occur on the bare ground away from the large plants and especially in the wadis. Steppe and desert formations are of the open type.
3. Temperate Districts.—Temperate districts are characterized by forests of deciduous trees and of coniferous trees, the latter being of different species from those of the warm temperate districts, but frequently of the same plant form. The identity of plant form of many of the conifers of both temperate and of warm temperate districts is probably a matter of phylogenetic and not of ecological importance.
Britain is fairly typical of the west European district. In these islands, we find forests[1] or woods of oak (Quercus Robur and Q. sessiliflora), of birch (Betula tomentosa), of ash (Fraxinus excelsior, and of beech (Fagus sylvatica). In central Scotland, forests occur of Pinus sylvestris, and, in south-eastern England, extensive plantations and self-sown woods occur of the same species.
Just as in the Mediterranean region, the degeneration of forests has given rise to maquis and garigues, so in western Europe, the degeneration of forests has brought about different types of grassland, heaths, and moors.
4. Cold Temperate and Frigid Districts.—In the coldest portion of the north temperate zone, forests of dwarfed trees occur, and these occasionally spread into the Arctic region itself (Schimiper, 1904: 685). Schimper distinguishes moss tundra, Polytrichum tundra, and lichen tundra; and the lichen tundra is subdivided into Cladonia tundra, Platysma tundra, and Alectoria heath. Where the climate is most rigorous, rock tundra occurs (p. 685).
The types of vegetation (tropical forests, sclerophyllous forest, temperate forests, tundra, &c.) thus briefly outlined are groups of Schimper's “climatic formations.” Such groups are interesting in that they are vegetation units whose physiognomy is, in a broad sense, related more to climatic than to edaphic conditions. For example, Schimper, after describing the sclerophyllous woodland of the Mediterranean district and of the Cape district, says: “The scrub of West and South Australia in its ecological aspect resembles so completely the other sclerophyllous formations that a description of it must seem a repetition.” This resemblance, however, only has reference to the general aspect or physiognomy of the vegetation and to the plant forms: the floristic composition of the various sclerophyllous—and other physiognomically allied—associations in the various geographical districts is very different; and indeed it is true that, just as the general physiognomy of plant associations is related to climate, so their floristic composition is related to geographical position. Hence, in any cosmopolitan treatment of vegetation, it is necessary to consider the groups of plant communities from the standpoint of the climatic or geographical district in which they occur; and this indeed is consistently done by Schimper. Finally, within any district of constant or fairly constant climatic conditions, it is possible to distinguish plant communities which are related chiefly to edaphic or soil conditions; and the vegetation units of these definite edaphic areas are the plant formations of some writers, and, in part, the “edaphic formations” of Schimper.
When a district like England is divided into edaphic areas, a general classification such as the following may be obtained:—
1. Physically and physiologically wet habitats, with the accompanying plant communities of lakes, reed swamps, and marshes.
2. Physically wet but physiologically dry habitats,[2] with the accompanying plant communities of fens, moors, and salt marshes.
3. Physically and physiologically dry habitats, with the accompanying plant communities of sand dunes and sandy heaths with little humus in the soil.
4. Habitats of medium wetness, with the accompanying plant communities of woodlands and grasslands. This class may be subdivided as follows:—
a. Habitats poor in mineral salts, especially calcium carbonate, often rich in acidic humous compounds, and characterized by oak and birch woods, siliceous pasture, and heaths with much acidic humus in the sandy soil.
b. Habitats rich in mineral salts, especially calcium carbonate, poor in acidic humous compounds, and characterized by ash woods, beech woods, and calcareous pasture.
Ecological Adaptations.—It is now possible to consider the ecological adaptations which the members of plant communities show in a given geographical district such as western Europe, of which England of course forms a part. In the present state of knowledge, however, this can only be done in a very meagre fashion; as the effect of habitat factors on plants is but little understood as yet either by physiologists or ecologists.
Hydrophytes and hemi-hydrophytes (aquatic plants).—Of marine hydrophytes, there are, in this country, only the grass-wracks (Zostera marina and Z. nana) among the higher plants. Even these species are sometimes left stranded by low spring tides, though the mud in which the are rooted remains saturated with sea-water. Although many plants typical of fresh water are able to grow also in brackish water, there are only a few species which appear to be quite confined to the latter habitats in this country. Such species perhaps include Ruppia maritima, R. spiralis, Zannichellia maritima, Z. polycarpa, Potamogeton interruptus ( = P. flabellatus), and Naias marina.
In freshwater lakes and ponds, especially if the water is stagnant, aquatic plants are abundant. Aquatic vegetation may be conveniently classified as follows:—
Aquatic plants with submerged leaves: Chara spp., Naias spp., Potamogeton pectinatus, Ceratophyllum spp., Myriophyllum spp., Hottonia palustris, Utricularia spp.
Aquatic plants with submerged and floating leaves: Glyceria luitans, Ranunculus peltatus, Nymphaea (Nuphar) lutea, Callitriche stagnalis, Potamogeton polygonifolius.
Aquatic plants with floating leaves: Lemna spp., Hydrocharis Morsus-ranae, Castalia (Nymphaea) alba.
Aquatic plants with submerged leaves and erect leaves or stems: Sagittaria sagittifolia, Scirpus lacustris, Hippuris vulgaris, Sium latifolium.
Aquatic plants with erect leaves or stems (reed swamp plants): Equisetum palustre, Phragmites communis, Glyceria aquatica, Carex riparia, Iris Pseudacorus, Rumex Hydrolapathum, Oenanthe fistulosa, Bidens spp.
Marsh plants: Alopecurus geniculatus, Carex disticha, Juncus spp., Caltha palustris, Nasturtium palustre.
In many aquatic plants, the endosperm of the seed is absent or very scanty. The root-system is usually small. Root-hairs are frequently missing. The submerged stems are slender or hollow. Strengthening tissue of all kinds (and sometimes even the phloem) is more or less rudimentary. The stems are frequently characterized by aeration channels, which connect the aerial parts with the parts which are buried in practically airless mud or silt. Submerged leaves are usually filamentous or narrowly ribbon shaped, thus exposing a large amount of surface to the water, some of the dissolved gases of which they must absorb, and into which they must also excrete certain gases. Stomata are often absent, absorption and excretion of gases in solution being carried on through the epidermal layer. Chloroplastids are frequently present in the epidermal cells, as in some shade plants. Very few aquatic plants are pollinated under water, but this is well-known to occur in species of Zostera and of Naias. In such plants, the pollen grains are sometimes filiform and not spherical in shape. In the case of aquatic plants with aerial flowers, the latter obey
