Fig. 9.4-4. Longitudinal section of digestive gland of the insectivorous plant sundew (Drosera). These are extremely complex glands with a vascular supply, transfer cells (not distinguishable by light microscopy), tracheary elements and secretory cells. The secretory product -- sticky material that catches the insects and digestive enzymes -- are secreted by the outer layer of densely-cytoplasmic glandular cells. The secretion surrounds the head as a large viscous drop of material. It is prevented from seeping back into the gland by Casparian strips (not visible here) on all the radial walls of the outermost cells. Without this protection, the digestive enzymes could diffuse through the walls, enter back into the gland and digest the unprotected non-glandular cells. For more details, see pages 165 and 166 and Fig. 9.28 in Plant Anatomy (Mauseth).