from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Botany The central portion of an ovule in which the embryo sac develops; the megasporangium.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The tissue which surrounds and protects the embryo and lies inside of the integuments.
from the GNU version of the Collaborative International Dictionary of English
- n. See nucleus, 3 (a).
from The Century Dictionary and Cyclopedia
- In botany, the body of the ovule containing the embryo-sac; the nucleus of the ovule.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. central part of a plant ovule; contains the embryo sac
The nucellus is a cellular tissue enveloping one large cell, the embryo-sac or macrospore.
The plants included are, however, mainly well-established parasites, and the absence of nucellus is only one of those characters of reduction to which parasites are liable.
In a species of _Allium_, embryos have been found developing in the same individual from the egg-cell, synergids, antipodal cells and cells of the nucellus.
In some cases the embryo or the embryo-sac sends out suckers into the nucellus and ovular integument.
As the development of embryo and endosperm proceeds within the embryo-sac, its wall enlarges and commonly absorbs the substance of the nucellus (which is likewise enlarging) to near its outer limit, and combines with it and the integument to form the _seed-coat_; or the whole nucellus and even the integument may be absorbed.
In some plants the nucellus is not thus absorbed, but itself becomes a seat of deposit of reserve-food constituting the _perisperm_ which may coexist with endosperm, as in the water-lily order, or may alone form a food-reserve for the embryo, as in _Canna_.
The development of the ovule, which represents the macrosporangium, is very similar to the process in Gymnosperms; when mature it consists of one or two coats surrounding the central nucellus, except at the apex where an opening, the micropyle, is left.
Insemineae include forms in which the nucellus is not developed, and therefore there can be no seed.
Ovular characters determine the grouping in the Dicotyledons, van Tieghem supporting the view that the integument, the outer if there be two, is the lamina of a leaf of which the funicle is the petiole, whilst the nucellus is an outgrowth of this leaf, and the inner integument, if present, an indusium.
(Liliaceae) polyembryony results from an adventitious production of embryos from the cells of the nucellus around the top of the embryo-sac.