problem of avoiding submergence; and the stomata are confined to the upper side, which alone is in contact with the atmosphere. Those water plants which raise their leaves into the air, on the other hand, have leaves of a variety of shapes, which in most respects approach those of land plants. An interesting progression of leaves illustrating all three stages may be watched in spring in the Arrow-head (_Sagittaria sagittifolia_). The first leaves produced are entirely submerged, and conform to the usual ribbon shape and delicate texture. Those which follow float on the surface. In them the lower part is contracted into a flaccid winged petiole, the upper part being expanded into an oblong floating blade with a waxy surface to keep the leaf dry on the upper side. These in turn give way to the characteristic aerial arrow-shaped leaves of summer, which approach in character the leaves of land plants, and are borne on stout, stiff petioles capable of resisting wind and wave. Coming now to FLOWERS, it is possible here to refer only to a few macroscopic or “naked-eye” characters and modifications; the full study of the flower and its essential functions being a matter for the laboratory and the high-power microscope, as very minute structures are involved. As briefly described in Chapter IV., flowers are groups of modified leaves arranged mostly very close together at the ends of branches, the tip of the shoot being often expanded into a _receptacle_ (very well seen in the Compositæ--_e.g._, Dandelion) for the accommodation of the crowded floral leaves. Just as the foliage leaves have become modified to carry on to the best advantage the process of assimilation, so the different series of floral leaves are specially adapted to their several functions. The sepals, which compose the _calyx_, having usually a protective rôle, in most cases enclose the young flower with a tough envelope; they usually retain their primitive green colour, and take part in the process of assimilation. They may drop off as the flower opens (_e.g._, Poppy), or wither as the petals wither, or remain fresh until the fruit is ripe. Sometimes, as in many Ranunculaceæ (compare _Anemone_, _Caltha_, _Helleborus_), they take on the advertising rôle usually assigned to the petals, being large and coloured, while the petals themselves are minute. In the Monocotyledons they usually join with the petals in adorning the flower. The next whorl, lying inside (that is, above) the sepals, is formed of petals, constituting the _corolla_. The connection of colour and form of petals with the visits of insects, and their relative insignificance in wind-pollinated flowers, has already been referred to (p. 81). The marvellous variety of colour and form observable in the corolla has for its main object the attracting of insects to the flower. The petals have departed much farther from the ordinary leaf-form than the sepals. They assume brilliant hues of every tint, the pigment being due either to