Structure, Development and Cytochemistry of Mucilage-secreting Hairs in the Moss Timmiella barbuloides (Brid.) Moenk

Mucilage-secreting hairs of the moss Timmiella barbuloides develop at the base of young leaves and differentiate three to four basal cells which form a non-secretory stalk, and five to nine distal secretory cells. In both cell types the external walls are covered by a thin cuticle, and connecting walls are traversed by numerous plasmodesmata. Young secretory cells develop extensive stacks of rough endoplasmic reticulum and numerous dictyosomes. Mucilage secretion is granulocrine and involves two morphologically and cytochemically distinct vesicle types: smaller, dictyosome-derived vesicles which are positive to the periodic acid/thiocarbohydra-zide/silver proteinate (PATAg) test for carbohydrates; and larger, PATAg-negative vesicles of uncertain origin. Coated vesicles arising from dictyosomes are also present but they do not seem to participate in secretion. In the first phase of activity, the smaller vesicles discharge their contents by exocytosis and the secretion accumulates beneath the cuticle which becomes separated from the cell walls. Later, the massive discharge of the larger vesicles is followed by the bursting of the cuticle and the release of mucilage from the cells. Concomitant to this, extensive invaginations of the plasmalemma are formed along the external walls. The mucilage is rich in carbohydrates but no appreciable amounts of protein were detected. β-Glycerophosphatase activity is found to be associated with dictyosomes and endoplasmic reticulum in actively secreting cells. Neither plastids nor endoplasmic reticulum appear to participate in secretion directly. Following the release of mucilage, the hair cells degenerate.