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Once the pollen grain is recognized and hydrated, the pollen grain germinates to grow a pollen tube. [11] There is competition in this step as many pollen grains may compete to reach the egg. The stigma plays a role in guiding the sperm to a receptive ovule, in the case of many ovules. [11]
Embryos that result from this mechanism can germinate into fully functional plants. As mentioned, the embryo results from a single pollen grain. Pollen grains consists of three cells - one vegetative cell containing two generative cells. According to Maraschin et al., androgenesis must be triggered during the asymmetric division of microspores ...
Once the pollen grain lands on the stigma of a receptive flower (or a female cone in gymnosperms), it takes up water and germinates. Pollen germination is facilitated by hydration on the stigma, as well as by the structure and physiology of the stigma and style. [2] Pollen can also be induced to germinate in vitro (in a petri dish or test tube ...
The pollen grain begins to germinate (unless a type of self-incompatibility that acts in the stigma occurs in that particular species and is activated), forming a pollen tube that penetrates and extends down through the style toward the ovary as it follows chemical signals released by the egg.
The best studied mechanisms of SI act by inhibiting the germination of pollen on stigmas, or the elongation of the pollen tube in the styles. These mechanisms are based on protein-protein interactions, and the best-understood mechanisms are controlled by a single locus termed S, which has many different alleles in the species population.
If the pollen is compatible it will germinate and begin to grow. [5] The ovary releases chemicals that stimulates a positive chemotropic response from the developing pollen tube. [6] In response the tube develops a defined tip growth area that promotes directional growth and elongation of the pollen tube due to a calcium gradient. [5]
Polyploidy and polyteny can also be seen sometimes. The tapetum's unusually large nuclear constitution helps it provide nutrients and regulatory molecules to the forming pollen grains. The following processes are responsible for this: Endomitosis; Normal mitosis not followed by cytokinesis; Formation of restitution nuclei; Endoreduplication
An endosperm is formed after the two sperm nuclei inside a pollen grain reach the interior of a female gametophyte or megagametophyte, also called the embryonic sac.One sperm nucleus fertilizes the egg cell, forming a zygote, while the other sperm nucleus usually fuses with the binucleate central cell, forming a primary endosperm cell (its nucleus is often called the triple fusion nucleus).