Question

Enlist pollination control mechanisms.

Answer 1

In flowering plants, pollen grains germinate to form pollen tubes that transport male gametes (sperm cells) to the egg cell in the embryo sac during sexual reproduction. Pollen tube biology is complex, presenting parallels with axon guidance and moving cell systems in animals. Pollen tube cells elongate on an active extracellular matrix in the style, ultimately guided by stylar and embryo sac signals. A well-documented recognition system occurs between pollen grains and the stigma in sporophytic self-incompatibility, where both receptor kinases in the stigma and their peptide ligands from pollen are now known. Complex mechanisms act to precisely target the sperm cells into the embryo sac. These events initiate double fertilization in which the two sperm cells from one pollen tube fuse to produce distinctly different products: one with the egg to produce the zygote and embryo and the other with the central cell to produce the endosperm.
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Answer 2

Dichogamy -Flowers bisexual but anthers and stigma ripen at different times

a) protandry - The stamens ripen earlier than the stigma E.g Crocus , Clerodendron

b) protogyny - stigma ripen first Eg Solanum ,Mirabilis , Scrophularia

Herkogamy - Flowers are bisexual but thete may be some barrier standing between the stamens and the pistil of the same flowrr.Ahood covering the stigma is a common form.of barrieras in Iris Anthers and stigma may lie at same distance from one another or vice versa -anthers exerted and style inserted . The stamens and style move away from each othet as in Clerodendron thomsonae

Hetrostyly - some plants bear two different forms of flowrrs.One type bears long stamen and a short style and the other type bears short stamens and long style called dimorphic hetrostyly E.g Primrose

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