nucellus

The nucellus is a cellular tissue enveloping one large cell, the embryo-sac or macrospore.

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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.

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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.

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(Liliaceae) polyembryony results from an adventitious production of embryos from the cells of the nucellus around the top of the embryo-sac.

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Insemineae include forms in which the nucellus is not developed, and therefore there can be no seed.

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In some cases the embryo or the embryo-sac sends out suckers into the nucellus and ovular integument.

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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.

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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_.

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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.

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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.

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