EEB 337H - Families of Vascular Plants

Lecture notes, 3 November 2009

Selected Angiosperm Synapomorphies

Double Fertilization and Endosperm


Double fertilization consists of fertilization of two or more nuclei in the angiosperm megametophyte.

The first fertilization gives rise to the embryo.
Fertilization of the egg cell by one of the sperm nuclei from the pollen tube (microgametophyte) results in formation of a single-celled zygote that, following cell divisions, develops into the embryo sporophyte within the seed.
The second fertilization gives rise to the endosperm.
Fertilization of the polar nuclei (or nucleus, if the polar nuclei have already fused) by the other sperm nucleus results in formation of an endosperm nucleus. Proliferation of the endosperm may proceed by way of free nuclear divisions, followed by cellularization.
Double fertilization occurs in all angiosperms.
However, the composition of the megagametophyte, and hence the ploidy level of the endosperm, may vary. Thus, for example, in Nuphar (spatterdock) the megagametophyte consists of four haploid nuclei: three making up the egg apparatus (1 egg + 2 synergids), and a fourth, the polar nucleus. As a result, after double fertilization occurs both the embryo and the endosperm are diploid. The situation shown at the right, which results in formation of a diploid embryo and triploid endosperm, is by far more common. Nevertheless, in some other cases, the composition of the megagametophyte is such that the endosperm may have a still higher ploidy level. Finally, in some cases, the megagametophyte may develop not from one, but rather from two or more of the megaspores.

 

Double ferilization - Wikipedia image
[Back to TOP] (above) A, a flower on the female inflorescence of Zea mays in longitudinal section, showing the ovary enclosing a single ovule within which lies the megagametophyte ("embryo sac"). B, events within the megagametophyte during and after double fertilization: (a) discharge of the sperm nuclei from the pollen tube into megagametophyte; (b) nuclear fusions (double fertilization) giving rise to a 2x zygote and a 3x endosperm nucleus; (c) a later stage, at a time when a multicellular embryo has formed from the zygote, and the endosperm is cellular. Note that in this diagram only the egg, and not the entire egg apparatus (egg + synergids), is shown.
 

Kiesselbach, T. A. The structure and reproduction of corn. Nebr. Agric. Exp. Stn Annu. Rep. 161, 196 (1949), cited at http://en.wikipedia.org/wiki/Double_fertilization.


Endosperm provides nutrients to the embryo during its development and (or) germination.

Endosperm may persist, so as to be present in the mature seed, as in the Poaceae.
As in many other monocots (e.g. the coconut palm) the seeds of the Poaceae contain abundant, starchy endosperm that will provide the energy and nutrients required for germination and seedling establishment.
The seeds of many non-monocots likewise may contain more or less abundant endosperm at maturity.
For example, the bulk of a nutmeg, i.e. the seed of the nutmeg, Myristica fragrans (Magnoliales, Myristicaceae), is mostly endosperm. Such endosperm-filled seeds are referred to as albuminous seeds.
The seeds of many other plants lack endosperm at maturity.
In these plants the endosperm forms but then is consumed during the maturation of the embryo, so that at maturity the seed contains only the tissue of the embryo. The seeds of the Fabaceae are examples of such exalbuminous seeds.

 

Triticum embryo and endosperm
[Back to TOP]

(above) Triticum fruit in longitudinal section.

 

Image © 2008 Nancy G. Dengler

 


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