FINE STRUCTURE OF THE MERISTEMATIC CELLS OF TAKAKIA LEPIDOZIOIDES HATT. ET H. INOUE (TAKAKIOPHYTA)

Abstract

  The fine structure of the potentially mitotic, undifferentiated cells of the apical meristems and regenerants of Takakia lepidozioides are compared, and the effects of kinetin and colchicine on regenerant cell substructure is investigated. Interphasic, meristematic cells in the apices of both leafy shoots and stolons are each characterized by a prominent central nucleus that occupies 40-50% of the cross-sectional area of the cell and one (early interphase) or two (later interphase) large, starch-filled plastids. The nucleus possesses a single large nucleolus, with a well defined outer granular zone, and several scattered regions of heterochromatin. In both young leaf cells and epidermal cells of the stem and stolon the single plastid of early interphase lies between the nucleus and the inner cell wall. The plastid divides by constriction at its midpoint; the two daughter plastids then migrate to lateral positions on either side of the interphase nucleus. The nucleus remains basically spherical although it may be distended slightly towards the dividing plastid. In stolon epidermal cells only, large, dense bodies lie between the nucleus and outer cell wall. Algal-like plasmodesmata are well-developed in both apices. Meristematically active cells of stem regenerants, in contrast, are each characterized by a central to subbasal nucleus, numerous, small, peripheral plastids, prominent vacuoles and osmiophilic lipid bodies that line all inner walls. Addition of kinetin to the regenerant culture medium modifies plastid/nuclear associations and causes membrane proliferation and vacuolation of lipid bodies. Post-treatment of kinetin-grown regenerants with colchicine results in disorientation of organelles, increase in nuclear dimensions and simplification of the plastid thylakoid system. That Takakia is an archaic archegoniate, only remotely related to other bryophytes, is supported by these findings.