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It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. Other examples of endo that forms the bulk of the edible portion are coconut "meat" and coconut "water", and corn. Some plants, like the orchid, lack endo in their seeds.
Origin of endo
Ancestral flowering plants have seeds that have small embryos and abundant endo, and the evolutionary development of flowering plants tends to show a trend towards plants with mature seeds with little or no endo. In more derived flowering plants the embryo occupies most of the seed and the endo is non developed or consumed before the seed matures.
Endo is formed when the two [[| nuclei]] inside a pollen grain reach the interior of an embryo sac or female gametophyte. One nucleus fertilizes the egg, forming a zygote, while the other nucleus usually fuses with the two polar nuclei at the center of the embryo sac, forming a primary endo cell (its nucleus is often called the triple fusion nucleus). This cell created in the process of double fertilization develops into the endo. Because it is formed by a separate fertilization, the endo constitutes an organism separate from the embryo.
One primitive species of flowering plant, Nuphar polysepala, has endo that is diploid, resulting from the fusion of a pollen nucleus with one, rather than two, maternal nuclei. It is believed that early in the development of angio lineages, there was a duplication in this mode of reproduction, producing seven-celled/eight-nucleate female gametophytes, and triploid endos with a 2:1 maternal to paternal genome ratio.
There are three types of Endo development:
Nuclear endo formation - where repeated free-nuclear divisions take place; if a cell wall is formed it will form after free-nuclear divisions. Commonly referred to as liquid endo. Coconut juice is an example of this. There are the two major hypothesis:
- The double fertilization initially used to produce two identical, independent embryos ("twins"). Later these embryos acquired different roles, one growing into mature organism and another merely supporting it. Any of these two embryos are capable of filling in the seed but normally only one develops further (the other eventually aborts).
- Becoming triploid or polyploid are later evolutionary steps of this "primary gametophyte". Nonflowering seed plants (conifers, cycads, Ginkgo, Ephedra) form a large homozygous female gametophyte to nourish the embryo within a seed.
The role of endo in seed development
In some groups (e.g. grains of the family Poaceae) the endo persists to the mature seed stage as a storage tissue, in which case the seeds are called "albuminous" or "endoous", and in others it is absorbed during embryo development (e.g., most members of the family Fabaceae, including the common bean, Phaseolus vulgaris), in which case the seeds are called "exalbuminous" or "cotyledonous" and the function of storage tissue is performed by enlarged cotyledons ("seed leaves"). In certain species (e.g. castor bean, Ricinis communis) and others (including grains, such as wheat and corn) store mainly starches. Orchid seedlings are mycoheterotrophic in their early development. Instead the nucellus produces a nutritive tissue termed "peri". The endo of some species is responsible for seed dormancy. Endo also mediates the transfer of nutrients from the mother plant to the embryo, it acts as a location for gene imprinting, and is responsible for aborting seeds produced from genetically mismatched parents. In angios, the endo contain hormones such as cytokinins, which regulate cellular differentiation and embryonic organ formation.
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