a single layer of slightly larger cells called the epidermis. The common white clover is a good illustration. The nectar gland is on the inner side at the bottom of the tube or cylinder formed by the cohesion of the filaments of the stamens, at the base of the free stamen. According to Cook, it is a little mound composed of 6 to 8 layers of small cells, which are easily distinguished from the larger cells of the adjacent tissues. The epidermis consists of a single layer of cells. The nectar-secreting tissue often forms a ring around the base of the ovary, or it may line the base of the calyx tube, or again it may develop as a cushion on top of the ovary. Sometimes it is at the bottom of a little cup or pit, as in the buttercups. In basswood, nectar is yielded by many little pegs projecting above the nectary tissue. In the mustard family and in the catalpa there are numerous pegs or knobs on the nectary. In the bindweeds the nectary consists of five scales at the base of the ovary. In the day lily the nectary is a long narrow pore, branched at its lower end and lined with large oblong cells. Nectaries are often white, but they may be of a deep orange color. It would require an entire chapter to describe the many remarkable ways in which nectaries are protected from rain and undesirable visitors. For example, on the petals of the common garden flower love-in-the-mist, there is a little bowl with a scale-like cover which the honeybee raises when it seeks the nectar, and which falls back into place after the departure of the bee.
THE PHYSIOLOGY OP NECTAR SECRETION
The passage of nectar through the epidermis of the nectary in the case of many flowers offers little difficulty. In Spiraea, thornbush, petunia, pumpkin, and many other blossoms there are numerous stomata (openings in the epidermis, protected by guard cells) through which the nectar comes to the surface. Beneath each stoma, or opening, there is a small chamber in which the nectar accumulates before it passes out of the nectary. The stomata may be distributed uniformly over the surface of the nectary, or they may be collected together in special spots. In the willows the nectaries have the form of pegs with a single large pore at the truncated end, through which escapes the colorless nectar. There are also nectaries which are covered with papillae, or single, elongated, thin-walled cells. These cells are not cutinized but consist wholly of cellulose, through which water and substances soluble in it readily pass. Nectar diffuses through the walls of the papillae and collects as drops upon them because of the outward pressure exerted within the distended cells of the nectary.
But not all nectaries have the outer cell-walls readily permeable by nectar. In many flowers the epidermis may be composed of two or three layers of cells, there may be no stomata, and the outer cell-walls may be cutinized. Cutin is a fatty substance which may almost entirely prevent the escape of water. The epidermis of aquatic plants contains little cutin; and for this reason, when exposed to the air, the water quickly escapes and they dry up in a few hours. On the other hand, plants growing in a dry situation usually have the epiderims heavily cutinized. When the time comes for the secretion of nectar the outer layers of such nectaries may swell and disintegrate, forming a gummy or gelatinous substance. With the destruction of the cutinized cells, the remaining cells of the epidermis, composed of cellulose, become readily permeable to nectar or other solutes. As sugar is readily derived from cellulose, it may be formed during the breaking down of the outer cells on the exterior of the nectary.
If the cells of the nectary are distended with water, the passage of nectar through the epidermis offers no great difficulty. In the absence of cutin, and where there is only one layer of cells, it occurs by diffusion; and when cutin is present, the epidermis is rendered permeable by the breaking-down of the outer cell-walls. But there is another membrane, lying within the cell, which it is much more difficult to