A Light and Electron Microscopic Study of Rhizoid Development in Polysiphonia Lanosa (L.) Tandy¹,²

SUMMARY The ontogeny of P. lanosa rhizoids was investigated by light and electron microscopy. A series of distinct, changes was recognized. Prior to penetration of the host, Ascophyllum nodosum , starch reserves were depleted and a large number of dictyosome vesicles accumulated in the cell. These appeared to have been discharged immediately prior to or during host, penetration with a concomitant increase in rhizoid wall thickness. Following initial penetration, the tip became highly convolute and dominated by ER, ribosomes, and mitochondria. Senescence was marked by almost complete, vacuolation of the cytoplasm, degeneration of most organelles, and the loss of color. These ultrastructural changes are correlated wth the habit of P. lanosa .