pass. The cell-walls of all living plant cells are lined with a thin, delicate membrane of protoplasm, known as the ectoplasm, which allows foods to enter the cell, but will not permit them to pass out and be wasted. In what way this living plasma membrane acts can not be fully explained, but it evidently performs a most useful service. As soon as it dies it loses the power of selection, and! nectar or cell sap can then readily escape from the cell. It acts only so long as it is living. How, then, does nectar pass through this living membrane and come to the surface of the nectary?
In order that a nectary may secrete nectar, its cells must be distended with water, or a solution of water and sugar. The nectar must exert a strong pressure outward, distending the elastic walls of the cells. If, owing to dry weather, there is not sufficient moisture in the soil, the leaves and young stems of plants will wilt or droop. They are rigid only when the cells, of which they are composed, are distended or swollen with water, or turgid, as it is termed by botanists. As has been pointed out by Sachs, the nectaries of plants secrete nectar only when the parts concerned are turgid. If the cells are flaccid no nectar is secreted. Since growth takes place only when the plant cells are turgid, flaccid cells are largely inactive. The advent of dry weather has often cut short the honey-flow. Plants with leaves and flowers drooping for want of water will not yield nectar. The tugor pressure in nectar cells may amount to as much as five atmospheres. Secretion in all probability begins by the forcing of nectar through the ectoplasm and epiderims, which are perhaps modified by the pressure. There would, of course, be less resistance on the free or outer side of the nectary than on the inner side, which is in contact with other plant tissues. As soon as the nectar comes to the external surface the water evaporates and a solution is formed more dense than that within the cells of the nectary; more nectar might then be drawn outward by osmosis. If nectar formed more rapidly than it escaped by osmosis, tugor pressure might manifest itself repeatedly. Secretion by floral nectaries ceases with pollination or soon after.
The quantity of nectar secreted differs greatly with different species of plants, wholly independent of external conditions. Nectaries are not all equally efficient. In some species small drops appear which are scarcely visible to the naked eye; in others the nectar forms a thin layer, appearing like a coat of varnish; in still others the drops flow together and collect in cups or tubes, which may be filled to overflowing. There is in the flower of the tropical orchid Coryanthes a collecting cup, into which nectar gradually trickles until it amounts to nearly an ounce. External conditions do not explain why two species yield very unequally under exactly the same conditions. Why is alfalfa a good honey plant only in the West, while white sweet clover yields well everywhere? Why is alsike clover a more reliable honey plant than white clover, or the bushy goldenrod than the early goldenrod, when both grow side by side and bloom at the same time? These differences depend upon the plant itself, or upon the chemical processes taking place within it.
It should not be forgotten that there are many kinds of glands besides nectaries. Water glands excrete large quantities of water, and many intermediate stages occur between them and nectaries. There are also resin glands, chalk glands, salt glands, slime glands, and the digestive glands of insectivorous plants. Internal glands occur in the leaves of the orange, Eucalyptus, and many other plants, which may not discharge their contents to the surface. Very little is known about the way in which each kind of gland forms the substance peculiar to it. Their origin is not clear, and their functions are often useless.
EXTERNAL FACTORS INFLUENCING NECTAR SECRETION
The principal external factors influencing nectar secretion are the soil, moisture and dryness, heat and cold, light, and altitude. The effects produced by these