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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).
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