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. [Illustration: Fig. 36. _Hippuris vulgaris_: 1, submerged leaf; 2, aerial leaf. × 130.] =171. Hydrophytic types.= Hydrophytes permit a fairly sharp division into three groups, based primarily upon the relation of the leaf surface to the two media, air and water. In submerged plants, the leaves are constantly below the water; in amphibious ones, they grow normally in the air. Floating plants have leaves in which the upper surface is in contact with the air, and the lower in contact with the water. Transpiration is at a maximum in the amphibious plant; it is reduced by half in the floating type, and is altogether absent in submerged plants. Aeration reaches a high development in amphibious and floating forms, but air-passages are normally absent from submerged forms except as vestiges. Photosynthesis is marked in the former, but considerably weakened in the latter. The vascular system, which attains a moderate development in the amphibious type, is considerably reduced in floating forms, and it is little more than vestigiate in submerged ones. [Illustration: Fig. 37. Floating leaf of _Sparganium angustifolium_. × 130.] 1. _The amphibious type._ Plants of this type grow in wet soil or in shallow water. The leaves are usually large and entire, the stem well developed, and the roots numerous and spreading. In the majority of cases the leaves are constantly above the water, but in some species the lower leaves are often covered, normally, or by a rise in level, and they take the form or structure of submerged leaves. This is illustrated by _Callitriche autumnalis_, _Hippuris vulgaris_, _Ranunculus delphinifolius_, _Proserpinaca palustris_, _Roripa americana_, _etc._ The epidermis has a thin cuticle, or none at all, and is destitute of hairs. The stomata are numerous and usually more abundant on the upper than on the lower surface. The palisade tissue is represented by one or more well-developed rows, but this portion of the leaf is regularly thinner than that of the sponge part. The latter contains large air-passages, or, in the majority of cases, numerous air-chambers, usually provided with diaphragms. The stems are often palisaded, and are characterized by longitudinal air-chambers crossed by frequent diaphragms, which extend downward through the roots.