SUSPENSOR DEVELOPMENT IN THE NUN ORCHID, PHAIUS-TANKERVILLIAE

The suspensor of the Nun orchid (Phaius tankervilliae) consists of a s ingle cell. which originates as follows: after fertilization, the zygo te divides oner, unequally, giving rise to a smaller terminal cell and a larger basal cell. At the two-celled stage, a prominent cortical ar ray of microtubules is present in the basal cell. The actin filaments are more centrally located within the cell having a perinuclear arrang ement, and some actin material is also present at the micropylar end o f the basal cell. After the basal cell divides one more time, the cell near the micropyle enlarges and differentiates into the suspensor cel l. The enlargement of the suspensor results primarily from vacuolation . Characteristic organization of the cytoskeletal elements can be foun d at the time of cell enlargement. The actin filaments are localized i n the cortical region of the cell, while the microtubules become inter nalized, forming a perinuclear array with extensions toward the poles of the cell. The final structural event in the development of the susp ensor is the extension of the suspensor cell through the inner integum ent into the outer integument. At this stage, a cortical array of micr otubules once again reappears. The cortical microtubules of the suspen sor near the embryo proper are arranged transversely while those near the tip portion of the suspensor are aligned parallel to the long axis of the cell. The actin filaments remain unchanged and appear as a net like framework located in the cell cortex. As the embryo matures, the cytoskeletal elements become disorganized and defined structures canno t be discerned.