Structure of Pistil

Structure of Pistil

         

After the pollination the pollen grains rest on stigma where they germinate. The interaction between pollen grains and the whole pistil determines the fate of pollen grains. As we know, pistil has three parts- stigma, style and ovary.

        

Structure of stigma –

The uppermost portion of the pistil, which receives pollen grains, is specialsed to catch and hold on to the pollen grains. Young stigma is made up of columnar tissue with as light depression in the center. Its surface can be divided into two zones-

     

(a)   Upper secretory zone –

Consists of epidermis and 3-4 layers of cells. These epidermal cells may further divide to form papillae. It also contains small cavities known as schizogeneous cavities, where the stigmatic exudates accumulate. This exudate is a phenolic substance with lipids, free sugars amino acids, proteins and peptides. These exudates may vary from species to species. It is highly viscous and refractive. This exudates is absent in dry stigmas.

     

(b) Lower storage zone –

contains the storage products.

Role of stigmatic exudates in pollen tube growth-

Helps in the adhesion of pollen grains on the stigmatic surface.

Increases the hydration of the pollen grains and induces germination.

Shows antimicrobial activity and hence prevents infections.

Hydrates the stigma.

Serves as nutrition for pollinating agents.

Provides nutrition to developing pollen tube.

Structure of style -

On the basis of structure style can be classified into three types-

Solid – It has a central strand of elongated specialized cells, the transmitting tissue, which guides the path of pollen tube growth. E.g. Most of the dicots.

Semi-solid- In this type, the stigma and the part of the style just below the stigma are solid. Below this, a canal is formed by dissolution of the transmitting tissue .E.g. Leguminosae.

Open style (hollow style) - A canal which begins below the stigma and runs down up to the base of the style. This canal is filled up by a mucilaginous substance. E.g. Monocots, Papavaraceae and Aristolochiaceae.

  

Structure of ovule –

When the path of pollen tube is directed to the ovule, then it may enter the ovule, then it may enter the ovule in three ways-

Porogamy- When pollen tube enters through micropyle.

Chalazogamy- When pollen tube enters by piercing the chalazal region. E.g. Casuarina.

Mesogamy- When pollen tube enters through the funiculus (Pistcia ) or through the integuments (Cucurbita).

        

 In some ovules a special structure called Obturator is present, which guides the pollen tube into the ovule.

Entry of the pollen tube into the embryo-sac-

         

The pollen tube always enters through micropylar region into the embryo-sac. Then, it enters into a synergid, which shows an increase in gibberlic acid. (Jensen & Ashton,1981). Here the pollen tube develops a pore, through which it’s contains are discharged, into embryo-sac

         The pollen tube discharge contains two sperms, vegetative nucleus and some cytoplasm. The two sperms are not male nuclei, but are true cells each with a plasma membrane of itself.