The most widely used classification of the nucellus relates to the position of the megasporocyte.
In crassinucellate ovules the nucellus is therefore a sporangiophore-sporangium complex (Herr, 1995).
Following the earlier literature and current conventions, Dahlgren and Clifford (1982) identified two characters relating to the micropylar region of the nucellus in monocotyledons: (1) presence or absence of parietal cell (sometimes called "tapetal cell," or Deckzellen) and (2) presence or absence of a multiseriate nucellar cap derived from cell divisions in the nucellar epidermis.
Family Nucellus Anarthriaceae Tenuinucellate Arecaceae Crassinucellate Bromeliaceae Crassinucellate Cannaceae Crassinucellate Cartonemataceae Crassinucellate Centrolepidaceae Tenuinucellate Commelinaceae Crassinucellate (tenuinucellate in Cyanotis) Costaceae Crassinucellate Cyperaceae Crassinucellate Dasypogonaceae Crassinucellate Ecdeiocoleaeeae Tenuinucellate Eriocaulaceae Tenuinucellate Flagellariaceae Crassinucellate Haemodoraceae Crassinucellate Hanguanaceae Unknown Heliconiaceae Crassinucellate Hydatellaceae Unknown Joinvilleaceae Unknown, but nucellar cap present (Fig.
In some taxa the distal part of the nucellus degenerates entirely before anthesis, but in many monocotyledons it remains intact, sometimes forming a layer of enlarged cytoplasmrich secretory cells, as in Tulbaghia (Fig.
The structure of the proximal end of the nucellus, adjacent to the chalaza, is variable and often includes modifications such as a hypostase or enlarged dermal cells.
In contrast to many eudicots, especially Asteridae, the proximal part of the nucellus at anthesis is often substantial in monocots (Maze & Bohm, 1973; Cave, 1975; Berg, 1978, 1996).
As Tilton (1980b) observed, the term "hypostase" originally denoted all modifications of the proximal part of the nucellus (e.