EMBRYO SAC

EMBRYO SAC Embryo Sac (F emale Gametophyte): Origin and Development (Plates 51 and 52): It is generally believed that one of the main features distinguishing angiosperms from phylogenetically lower groups of plants is the presence in them of a unique embryo sac and double fertilization. Based on the striking similarities in embryo sac structure (the so-called 'normal' or Polygonum-type embryo sac development) exhibited by the overwhelming majority of angiosperms and universal occurrence among them of double fertilization, it appears very likely that precisely these features could have evolved but once in the evolutional history of angiosperms (Schnarf, 1929, 1936; Poddubnaya-Amoldi, 1930, 1951; Maheshwari, 1937, 1948, 1950; Romanov, 1944). Taxonomists consider this to be an irrefutable argument for a strictly monophyletic origin of angiosperms (Boush, 1944; Grossheim, 1945; Zhukovskyi, 1949). As far as other special types of embryo sac found in some angiosperms are concerned, these are viewed as derivatives of the 'normal' type which is thought to be the original, primitive one (Schnarf, 1929; Poddubnaya-Amoldi, 1930, 1951; Maheshwari, 1937, 1948, 1950; Romanov, 1944). A few plant embryologists accept the possibility that some other types of embryo sac might have evolved independently but their line of reasoning is not sufficiently convincing (Joshi, 1938; Modilewski, 1953).