The Digestive Glands of Pinguicula: Structure and Cytochemistry

The digestive glands of the carnivorous genus Pinguicula have three functional compartments, (a) a basal reservoir cell, (b) an intervening cell of endodermal character and (c) a group of secretory head cells. The gland complex is derived from a single epidermal initial. The reservoir cell, which is rich in Cl− ions, is highly turgid before discharge; it is linked by plasmodesmata to the surrounding epidermal cells, and is ensheathed by a pectin-rich inner wall layer. The endodermal cell is bounded by a Casparian strip to which the plasmalemma is tightly attached; it contains abundant storage lipid and numerous mitochondria. The head cells of the developing gland have labyrinthine radial walls of the transfer-cell type, the ingrowths being composed of pectic polysaccharides. The bounding cuticle is discontinuous, although lacking well-formed pores. Mitochondria are numerous, with well-developed cristae; the plastids are large and ramifying, and invested by ribosomal endoplasmic reticulum. Dictyosomes are sparse, and where they occur, are associated with coated vesicles. Ribosomal endoplasmic reticulum is moderately abundant in the head cells, and so also are free ribosomes. Optical and electron microscopic localization methods indicate that the digestive enzymes are synthesized in the head cells and transferred both into the vacuoles and into the walls. There is no evidence of a granulocrine mode of secretion, and the transfer seems to be initially by direct perfusion through the plasmalemma. During the final phase of maturation of the head cells they suffer a form of autolysis, vacuoles, cytoplasm and wall becoming confluent as all of the membranes of the cell undergo dissolution. The gland head is thus, in effect, simply a sac of enzymes at the time of the ultimate discharge.