Digestive glands have both secretory and absorptive functions. They are considered a further development of nectaries, i.a. because in certain nectar glands it has been established that some of the nectar is reabsorbed. In Nepenthes, in both structural and functional terms, there is an almost smooth transition from the pitcher edge to the bottom zone between nectaries and digestive glands. At the top, a very watery nectar-like liquid is secreted, below, various enzymes are secreted with the water, and in the lower part, which is usually below the liquid level, there are actual digestive glands which, in addition to enzyme secretion, are also able to absorb decomposition products from the dissolved prey.

     All glands are obliquely sunk into the otherwise completely smooth inner wall and partially covered by a half-roof of epidermal cells. This makes the glands unsuitable as a foothold. In Nepenthes x mixta shown on this page, the digestive glands consist of several hundred cells, and there are approximately 25 glands per mm². 

     Digestive glands arise, except in Sarraceniaceae, from a single epidermal cell. The glands are constructed on the same ground plan with a multicellular secretory head separated from one or more basal cells (reservoir cells) by a number of endodermoid stalk cells (yellow dots, bottom figure). Endodermoid cells are characterized by a cutinized (lipid-impregnated) zone in the side walls of the cells (anticlinal walls) all the way around the cell, corresponding to the Casparian strip in the endodermis of the root. The cutinization causes a blockage of all transport in the cell wall, so that the cell membrane and cytoplasm gain control over what passes in and out of the gland. It is often a question of blocking a return flow through the cellulose wall (apoplast) to the interior of the plant of the products that the gland cells pump out. As special structural features of digestive glands, we can mention that the conductive tissue’s tracheids (red arrow) usually reach only a single cell’s distance from the basal cells, that the cuticle of the gland head is perforated so that secretions can escape unhindered and nutrients can enter unhindered, and that so-called transfer cells are often included, where the inner surface of the cell wall and thus the area of ​​the cell membrane is greatly enlarged.

The amount and nature of secreted enzymes varies from species to species, but all secrete proteolytic enzymes before the lid opens. In Nepenthes burkei, at least five types of proteinases with different molecular weights are produced (126, 64.6, 50.3, 35.5 and 31.5 kD), and the highest proteinase activity is found at pH 3. As the pitcher ages, enzyme secretion stops and the glands switch to their absorptive function. With age, the jug liquid is also diluted with rainwater, so the pH rises, and bacteria, fungi and protozoa play an ever-increasing role in the breakdown of the prey. In some species, the pitcher liquid becomes two-phase with most water and a relatively high pH in the upper phase, where the living microorganisms reside, and a highly viscous, often milky, more acidic and enzyme-containing lower phase.

H S Heide-Jørgensen, Jan 2024.