After landing on a receptive stigma, a pollen grain absorbs moisture and germinates; that is, it produces a pollen tube that extends down between the cells of the style toward the ovary.
The nucleus of the generative cell divides by mitosis and forms two sperms. Directed by a chemical attractant, possibly calcium, the tip of the pollen tube enters the ovary, probes through the micropyle (a gap in the integuments of the ovule), and discharges its two sperm near or within the embryo sac.
The events that follow are a distinctive feature of the angiosperm life cycle. One sperm fertilises the egg to form the zygote. The other sperm combines with the two polar nuclei to form a triploid (3n) nucleus in the centre of the large central cell of the embryo sac. This large cell will give rise to the endosperm, a food–storing tissue of the seed. The union of two sperm cells with different nuclei of the embryo sac is called double fertilisation. Double fertilisation ensures that the endosperm will develop only in ovules where the egg has been fertilised, thereby preventing angiosperms from squandering nutrients.
The tissues surrounding the embryo sac have prevented researchers from being able to directly observe fertilisation in plants grown under normal conditions. Recently, however, scientists have isolated sperm from germinated pollen grains and eggs from embryo sacs and have observed the merging of plant gametes in vitro (in an artificial environment). The first cellular event that takes place after gamete fusion is an increase in the cytoplasmic calcium (Ca2+) levels of the egg, as also occurs during animal gamete fusion. Another similarity to animals is the establishment of a block to polyspermy, the fertilisation of an egg by more than one sperm cell. Thus, maize (Zea mays) sperm cannot fuse with zygotes in vitro. In maize, this barrier to polyspermy is established as early as 45 seconds after the initial sperm fusion with the egg.
From Ovule to Seed After double fertilisation, each ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). As the embryo develops from the zygote, the seed stockpiles proteins, oils, and starch to varying extents, depending on the species. This is why seeds are such major sugar sinks. Initially, these nutrients are stored in the endosperm, but later in seed development in many species, the storage function of the endosperm is more or less taken over by the swelling cotyledons of the embryo.
Endosperm Development Endosperm development usually precedes embryo development. After double fertilisation, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. This liquid mass, the endosperm, becomes multicellular when cytokinesis partitions the cytoplasm by forming membranes between the nuclei. Eventually, these “naked” cells produce cell walls, and the endosperm becomes solid. Coconut “milk” is an example of liquid endosperm; coconut “meat” is an example of solid endosperm. The white fluffy part of popcorn is also solid endosperm.
In grains and most other monocots, as well as many eudicots, the endosperm stores nutrients that can be used by the seedling after germination. In other eudicots (including bean seeds), the food reserves of the endosperm are completely exported to the cotyledons before the seed completes its development; consequently, the mature seed lacks endosperm.