Stigma

The stigma, along with the style and ovary (often referred to as the stigma-style-ovary system), makes up the pistil, which is a portion of a plant’s gynoecium, or female reproductive organ. The stigma is made up of stigmatic papillae, which have pollen-receptive cells and form the distal section of the style or stylodia. These can be limited to the style’s apex or can cover a large area, especially in wind-pollinated species. 

Stigma and pollen

The stigma accepts pollen and the pollen grain germinates on the stigma. With varied hairs, flaps, or sculpturings, the stigma is tailored in diverse ways to collect and trap pollen. Pollen can be collected from the air (anemophily), through visiting insects or other animals (biotic pollination), or, in rare situations, from nearby water (hydrophily). The stigma can be long and slender, globe-shaped, or feathery. 

When pollen leaves an anther, it is usually quite dry. Stigma has been demonstrated to aid in the rehydration of pollen and the promotion of pollen tube germination. Stigma also ensures that the correct pollen species adhere to the stigma. Pollen discrimination can be aided by stigma, and some self-incompatibility reactions, which reject pollen from the same or genetically similar plants, including the interaction between the stigma and the pollen grain’s surface. 

Structure of stigma

The stigmatic papillae are connected to the ovary by the style, which is a small upward extension of the ovary. It is sometimes absent, in which case the stigma is referred to be sessile. The majority of the styles are tube-like and come in both long and short lengths. The style can be open (with few or no cells in the central part) or closed (with a mucilage-filled central canal). It’s also possible to close the style (densely packed with cells throughout). Most syncarpous monocots and some eudicots have open styles, whereas many syncarpous eudicots and grasses have closed (solid) styles with specialised secretory transmission tissue that connects the stigma to the ovary’s centre. This creates a nutrient-rich environment for pollen tube development. 

Dry stigma

The development of methods for obtaining water from the stigmatic epidermis has accompanied the emergence of dry stigmas in pollen. Brassica oleracea has been studied for the development of self-and cross-pollen on the stigmatic surface, with a focus on grain hydration. 

Attachment to the ovary

Terminal (apical), subapical, lateral, gynobasic, or subgynobasic are all possibilities. The most prevalent pattern is the terminal (apical) style position, which refers to the attachment at the ovary’s apex. The style rises to the side, somewhat below the apex, in the subapical pattern. Rosaceae have a lateral style that emerges from the side of the ovary. Boraginaceae have a gynobasic style that emerges from the base of the ovary or between the ovary lobes. Allium is characterised by subgynobasic styles. 

Pollination

To reach the ovules, pollen tubes expand the length of the style, and in some situations, self-incompatibility responses in the style prevent pollen tubes from fully developing. The pollen tube is directed to the micropyle of the ovule by the style in some species, including Gasteria. 

Functions of stigma

Plants require a means of fertilisation and reproduction to survive. Pollen is the male gametophyte utilised in plant reproduction. Pollen grains must be transmitted from the anther, where they are formed, to the pistil, which contains the female gametophytes, for fertilisation to take place. Pollen is frequently carried by the wind or by creatures like butterflies and bees. Pollen needs a place to land and enter to reach the female gametophyte inside the pistil. The stigma is the landing zone and entrance in a flower. It is really important for reproduction.

To boost their efficiency, most stigmas have hairs, flaps, and other surfaces that capture pollen. A waxy, sticky material coats the majority of stigmas. This moist fluid enables stigmas to perform a crucial function by rehydrating dried pollen, allowing the grains to enter the ovary and begin the fertilisation process. 

Conclusion

The portion of the pistil where pollen germinates is known as the stigma. It is very vital for reproduction. The stigmas are mostly covered with a waxy, sticky material. To boost their efficiency, stigmas have evolved to capture pollen. The wet component on the stigma permits the pollen to be re-hydrated, which is a valuable service. The pollen then prepares the grains for entry into the ovary and the fertilisation process to begin.