XVI International Botanical Congess

The nucleus in an immature egg is elliptical. As it develops, it becomes amoeboid and sends out sac-like evaginations with electron opaque aggregates. It differs from Dryopteris filix-mas ( Bell 1974), in absence of a sheet of nuclear material in the form of closed ring. As egg matures, nucleus transforms into long elliptical. Mitochondria remain elliptical or rounded, decreasing in number and dedifferentiating in structure, and then recover normally. The new umbo-mitochondria in Pteridium aquilium ( Bell and Muhlethaler 1964) have not been found. Plastids containing starch and thylakoids are variable throughout and decrease in member gradually. In the immature egg vesicles in varied shape are most remarkable. As egg matures, small rounded vesicles emerge in large number, distributed at the periphery of the egg cell and especially at the anterior with electron opaque aggregates, surrounded by double membrane . It is possible that these vesicles arise from plastids and that they contribute to formation of the new additional egg membrane. A distinct osmiophilic additional membrane covering the mature egg has also been observed , but absent in adjacent cells.