2. The virgate form. The leaves either fall off early or they are reduced to functionless scales. The stems are thin, erect, and rod-like, and are often greatly branched. They are heavily cutinized and palisaded, and the stomata are frequently in longitudinal furrows. This type is characteristic of the Genisteae; it is also found in Ephedra, many species of Polygonum, Lygodesmia, etc.
3. The rush form. In Heleocharis, many species of Juncus, Scirpus, and other Cyperaceae, the stem, which is nearly or completely leafless, is cylindrical and unbranched. It usually possesses also a thick cuticle, and several rows of dense palisade tissue.
4. The cladophyll form. In Asparagus the leaves are reduced to mere functionless scales, and their function is assumed by the small needle-shaped branches.
5. The flattened form. As in the preceding type, the place of the scale-like leaves is taken by cladophylls, which are more or less flattened and leaf-like. Ruscus is a familiar illustration of this form.
6. The thorn form. This is typical of many spiny desert shrubs, in which the leaves are lost very early, or, when present, are mere functionless scales. The stems have an extremely thick cuticle, and the stomata are deeply sunken, as a rule. Colletia and Holacantha are good examples of the type.
7. The succulent form. Plants with succulent stems such as the Cactaceae, Stapelia, and Euphorbia have not only decreased water loss by extreme reduction or loss of the leaves, and the reduction of stem surface, but they also offset transpiration by means of storage tissues containing a mucilaginous sap. The cuticle is usually highly developed and the stomata sunken. Thorns and spines are also more or less characteristic features.
Fig. 35. Polygonum bistortoides, a stable type: 1, mesophyll (chresard, 25%); 2, xerophyll (chresard, 3–5%). × 130.
170. Bog plants. Many of the xerophytic types just described are found in ponds, bogs, and swamps, where the water supply is excessive, and hydrophytes would be expected. The explanation that “swamp xerophytes” are due to the presence of humic acids which inhibit absorption and aeration in the roots has been generally accepted. As Schimper has expressed it, bogs and swamps are “physiologically dry”, i. e., the available water is small in amount, in spite of the great total water-content. Burgerstein (l. c., 142) has shown, however, that maize plants transpire, i. e., absorb, three times as much water in a solution of 0.5 per cent of oxalic acid as they do in distilled water, and that branches of Taxus in a solution containing 1 per cent of tartaric acid absorb more than twice as much as in distilled water. Consequently, it seems improbable that small quantities of humic acids should decrease absorption to the extent necessary for the production of xerophytes in ponds and bogs. Indeed, in many ponds and streams, where Heleocharis, Scirpus, Juncus, etc., grow, not a trace of acid is discoverable. Furthermore, plants with a characteristic hydrophytic structure throughout, such as Ranunculus, Caltha, Ludwigia, Sagittaria, etc., are regularly found growing alongside of apparent xerophytes. Many of the latter, furthermore, show a striking contrast in size and vigor of growth in places where they grow both upon dry gravel banks and in the water, indicating that the available water-content is much greater in the latter. Finally, many so-called “swamp xerophytes” possess typically hydrophytic structures, such as air-passages, diaphragms, etc. In spite of a growing feeling that the xerophytic features of certain amphibious plants can not be ascribed to a low chresard in ponds and swamps, a satisfactory explanation of them has been found but recently. This explanation has come from the work of E. S. Clements already cited, in which it was found that certain sun plants underwent no material structural change when grown in the shade, and that the same was true also of a few species which grew in two or more habitats of very different water-content. In accordance with this, it is felt that the xerophytic features found in amphibious plants are due to the persistence of stable structures, which were developed when these species were growing in xerophytic situations. When it is called to mind that monocotyledons, and especially the grasses, sedges, and rushes, are peculiarly stable, it may be readily understood how certain ancestral characters have persisted in spite of a striking change of habitat. Such a hypothesis can only be confirmed by the methods of experimental evolution, and a critical study of this sort is now under way.
