The Ligule of Isoetes lacustris: Ultrastructure, Mucilage Composition, and a Possible Pathway of Secretion

Mature ligules of Isoetes lacustris can be divided anatomicallyinto three ultrastructurally different regions. First, the basalregion contains large numbers of two types of protein bodies.Second, the cells of the sub-marginal region are characterizedby a well developed Golgi apparatus closely associated withthe tubular compartments of the endoplasmic reticulum. Third,the peripheral region consists of one to three layers of cellsshowing an extremely well developed rough asternal and smoothtubular endoplasmic reticulum (ER). The tubular ER compartmentsare frequently observed in close attachment to the plasmalemma.The outermost peripheral cells are covered with a mucilaginouslayer. The dry matter in the mucilage consists of 49 per centpolysaccharides and 22 per cent proteins. The polysaccharidefraction, analysed by ion exchange chromato-graphy, consistsmainly of glucose, arabinose, galactose and uronic acids. Theprotein fraction was analysed by SDS gel dectrophoresis andby high performance liquid-chromatographic separation of theamino acids. The analysis shows a protein pattern very similarto that of the peripheral ligule tissue. It is suggested, therefore,that the material of the external mucilage is secreted by theperipheral ligule cells. The secretional mechanism appears tobe a direct release of polysaccharides and proteins by the tubularcomponents of the ER. There is no indication of secretion viathe Golgi apparatus. Because of its high activity in proteinsynthesis and secretion, it is suggested that the ligule isa vestigial structure, which, in extinct genera, might havefunctioned as a digestive organ. Isoetes lacustris, endoplasmic reticulum, ligule, ultrastructure, Polysaccharide secretion, protein secretion