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. [Illustration: 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