XVI International Botanical Congess

Evolution from the crassinucellate to tenuinucellate ovule proceeded through a series of medionucellate ovules and involved a gradual reduction of the nucellus. A gradual changeover from bitegmy to unitegmy occurred, such that the integument came to be laid down in closer proximity to the ovular primordium apex. In a number of highly specialized taxa, this was accompanied by a shaping of the pachychalazal ovule. The evolution of mono- and dicotyledonous ovules appears to have proceeded along separate pathways. Among the monocots, along with crassinucellate ovules, medionucellate ovules came to be of wide-spread occurrence, typical tenuinucellate ovules are lacking. Moreover, they exhibit predominantly bitegmic ovules. The evolution of the monocotyledonous ovule involved only a reduction in massiveness of its structures rather than their complete obliteration. The integuments are often only two layers thick, with no integumentary tapetum normally being differentiated. Another distinctive feature of monocots is that among them there are practically no taxa with the cellular type of endosperm.