Fertilization

Fertilization is the act of fusion of male and female gametes; in angiosperms it takes the unique form of double fertilization. The full sequence of events runs: germination of the pollen grain into a pollen tube on the stigma; growth of the pollen tube through the style; its directed growth towards the ovule's micropyle; entry of the pollen tube into one of the two synergids of the embryo sac and discharge of the male gametes there; and finally syngamy and triple fusion. The whole span of events from pollen landing on the stigma to pollen-tube entry into the ovule is called pollen-pistil interaction, a dynamic recognition process that either promotes or inhibits germination and growth of a particular pollen grain. Many different pollen types land on a stigma in nature, but only compatible pollen actually germinates and achieves fertilization; the stigmatic surface (wet, via stigmatic fluid, or dry, via a surface pellicle) mediates this compatible/incompatible recognition through specific recognition-rejection protein interactions between pollen and stigma, and incompatibility can be interspecific (between different species) or intraspecific/self-incompatibility (between members of the same species). The first visible response of compatible pollen on the stigma is hydration, followed by release of pollen-wall proteins and germination, during which the pollen's entire cytoplasmic content moves into the emerging tube; growth is confined to the tube's tip, where the cytoplasm concentrates, while the older part of the tube is instead occupied by a vacuole sealed off by a callose plug, with the extreme tip appearing hemispherical and transparent (called the cap block) - once this cap block disappears, tube growth stops. Once through the stigma, the pollen tube enters the style, and how it grows there depends on which of three style types is present: hollow (open) style, common in monocots, with a canal running from stigma to style base lined by glandular transmitting-tissue cells that secrete mucilage (carbohydrates, lipids, esterases, acid phosphatases and compatibility-controlling proteins) over whose surface the tube grows; solid (closed) style, common in dicots, where a central core of elongated transmitting-tissue cells performs the same nutritive/compatibility function and the tube grows through the intercellular spaces between them; and semi-solid (half-closed) style, an intermediate form. There is some difference of scholarly opinion on whether hollow-style lining cells truly count as transmitting tissue in the same sense as solid-style transmitting tissue, though both perform the equivalent function. On reaching the ovary, the tube can enter the ovule by one of three distinct routes: porogamy, entering through the micropyle (the commonest route); chalazogamy, entering through the chalaza; and mesogamy, entering through the integument. Whichever route it takes, the tube always enters the embryo sac itself at the micropylar end, and always directly into one of the two synergids. Growth of the tube towards the ovary, ovule and embryo sac is guided by chemotropic substances; after travelling the full length of the style, the tube enters the ovary locule and is guided to the ovule's mi …