Ontogeny of external glands in the bladderwortUtricularia monanthos

Summary The development of external glands on traps and stolons ofU. monanthos has been studied using transmission electron microscopy. During early differentiation of the epidermis some cells remain narrow and develop a protuberance which subsequently divides into a terminal and a pedestal cell, with the remainder of the original cell forming the basal epidermal cell of the gland. The lateral wall of the pedestal cell soon becomes densely impregnated throughout its thickness, and this is followed by the formation of discontinuous cuticular deposits within the primary wall of the terminal cell. The outer wall of the terminal cell then usually undergoes extensive secondary wall thickening beginning with the formation of ingrowths which for a period characterize the cell as a transfer cell. Later, at the stage when traps begin capturing prey, these ingrowths are overlain by further layers of secondary wall material. Concomitantly, in the pedestal cell, wall ingrowths become fully differentiated on the outer transverse wall and persist throughout the remaining life of the gland.The function of external glands during early ontogeny is discussed. At the stage when the terminal cell is differentiated as a transfer cell it is suggested that the gland is mainly responsible for absorbing solutes from the external medium. Once traps commence capturing prey the gland may become modified for a rôle in water secretion, facilitated by the differentiation of the pedestal cell as a transfer cell, and by the formation of a thick outer wall in the terminal cell.