ADVERTISEMENTS:
In this article we will discuss about the composition and importance of exince in pollen and spore walls.
Composition of Exine:
Exine is mainly composed of sporopollenin. Apart from sporopollenin glycoproteins are present in the chambers formed by bacula in tectate pollen. It is important in pollen-stigma interaction. The presence of glycocalyces (sing, glycocalyx) is reported from exine.
Glycocalyx forms a fine layer on the outer surface of many cells. It is composed of polysaccharides, polypeptides or both. In some cells it forms a coating consisting of glycoproteins and proteoglycans. It enables a cell to adhere, protects the cell from dehydration, and helps in absorption and cell individual identity.
ADVERTISEMENTS:
It is reported from Asteraceae (Calendula officinalis), Lauraceae (Persea americana), Boraginaceae (Borago officinalis), Alismataceae (Echinodorus cordifolium), and Nymphaeceae etc. In Lauraceae and Boraginaceae glycocalyx is found to consist of radially arranged helical cylindrical units. These units are the receptors of sporopollenin deposition and this is revealed with the study by means of electron microscopy.
The intine contains cellulose at the inner part and pectin on the outer part. It also contains callose at the apertural region. Various proteins are present in it. The intine also contains polyuronids or a mixture of polyuronids and polysaccharides. The pollen tube wall is made up principally of callose [β (1-3) glucan] and α (1-3) arabinan.
Importance of Exine:
ADVERTISEMENTS:
(i) It protects the protoplasm from desiccation and abrasion during pollination. Harder exine than other parts resists deterioration more successfully.
(ii) It is involved in cell-to-cell recognition and carries the chemicals (e.g. glycoproteins) that recognize the compatible receptive stigma. The chemicals are highly specific and also help in adhesion between pollen and stigmatic surface.
ADVERTISEMENTS:
(iii) It helps to adhere to a pollinator.
(iv) Through it the pollen tube emerges during fertilization.
(v) It allows for the expansion and reduction of pollen size with changing humidity.
(vi) The surface details of exine are remarkably variable and it yields the most important characters for the identification of pollen and spores.
(vii) The physical strength of a pollen grain is obtained from it. It can withstand shock without permanent deformation. It is very flexible and resilient. Generally the shape of pollen does not change after long-term sediment underground, mainly due to the existence of exine.
Light microscopy was the only means to study exine. After 1950s with the introduction of scan electron microscope and transmission electron microscope the exine structure is analyzed and tested with the light microscopic observation. After 1990s atomic force microscopy seems to be the ideal method to investigate the exine substructure.
The morphological subunit of sporopollenin and their arrangement patterns in the exine of angiosperm and gymnosperm can be well studied from atomic force microscope (AFM) images. A few reports are available so far concerning the substructure of exine.
From the AFM images the exine is regarded as a complex unit where the morphological subunits are shaped as spheroid and tubule. They are arranged in helical pattern (ex. Nuphar luteum). The subunits may also be in the form of granules that form 3-dimensional lattice pattern in the exine (ex. Cedrus deodara).
ADVERTISEMENTS:
Sporopollenin makes the exine tough and renders pollen grains and spores resistant to decay. As a result pollen and spores survive in ancient sediments for at least 500 million years. Shape, sculpturing and apertures etc. of a grain remain unchanged after long term sediment underground. These properties of a pollen grain form the basis of palynology.
Palynology (n. Gr. paluno = to strew or sprinkle; L. pollen = flour, dust; Gr. logia = combining form denoting theory or science) can be defined as the collective study of spores of embryophytic plants and pollen of seed plants.
In a broad sense the study also includes a number of other acid-resistant microorganisms and palynomorphs and other biological materials that are studied by palynological techniques. Hyde and William in 1944 advocated the term palynology.
[The term palynology replaced the name ‘pollen analysis’. In Pollen Analysis Circular (No. 8, 1944) it was expressed to have a better name other than pollen analysis. The Circular was a cyclostyled research bulletin. The Editors were Professor Paul B. Sears of Oberlin College, Ohio, Mr. H. A. Hyde of the Natural Museum of Wales and Mr. D. A. Williams of Llandough Hospital, Cardiff. In the October issue of the Circular the term ‘palynology’ was suggested. ‘It is hoped that the sequence of consonants p-l-n (suggesting pollen, but with a difference) and the general euphony of the new word will commend it.’]
