In this article we will discuss about the layers of the spore and pollen wall.
Sporoderm stratification (Erdtman’s concept):
Sporoderm (L. sporodermis), i.e. the spore and pollen wall is composed of distinct layers or subdivisions, the sequence of which is as follows from outside inwards in a fresh pollen grain and spore.
Sclerine (Gr. Scleros means hard-Fig. 4.1 A & B):
Sclerine is the peripheral layer of sporoderm excluding intine. It is entirely, or chiefly, hard (sclerodermic) and preserved in fossil spores. This layer has two well-defined subdivisions- perine and exine. The term can be used when there is doubt regarding the presence of perine, i.e. sporoderm excluding intine.
Perine (perisporium, L. perinium, Syn. perispore):
Perine is the outermost layer of sporoderm and more or less sculptured like sexine. It is chiefly present in spores of ferns and mosses. It is also reported from gymnosperm pollen, e.g. Taxodium. This layer is less resistant to certain chemicals and decay, e.g. Psdotum. This layer is usually wrinkled, folded in various ways and envelopes the mature spore in a loose manner.
Perine often cannot be differentiated from the sexine in fossil material. In paleontological considerations, the exterior parts of exine that form a natural separate surface are considered as perispore. It is interpreted as an additional wall layer external to exine. Sometimes it is difficult to ascertain whether a certain stratum is a part of perine or sexine.
In such cases the term sclerine is used as a comprehensive term for perine and exine. So the adjective ‘sclerinous’ can be applied to the layer which may be perinous, exinous or both of the two. Similarly sometimes it becomes difficult to decide whether the supratectal element is of perinous or sexinous nature.
Such cases are described as sculptine. It is used as a neutral term and signifies that the sculptured element may be a part of either perine, sexine or both. The term sculptine is used to describe the sculptural layer of sclerine.
The term perine is usually used in conjunction with exine and intine whereas the synonym perispore is used with exospore and endospore. The term exospore and endospore are homologous to exine and intine respectively. Mycologists use the two terms to describe spores that are formed outside and inside respectively of a sporangium. Perine, in contrast to exine, is less resistant to decay and acetolysis.
[Acetolysis is a procedure followed by palynologists to stain pollen present in peat and anther. Erdtman introduced the practice of acetolysing pollen grains by treating the materials in a mixture of nine parts of acetic anhydride and one part of concentrated sulphuric acid. The latter is a catalyst and desiccating agent. After acetolysis pollen grains stain yellow to orange colour. Sporopollenin remains unscathed and exine sculpturing becomes clearer to study.]
Exine (exospore; exosporium; L. exinium-Figs. 4.2, 4.1C & D, 4.3):
Exine is the outermost layer of angiosperm pollen where perine is absent. Exine is not uniformly thick throughout the circumference. It is thinner at apertural region. This layer has two well-developed subdivisions – sexine and nexine.
Sexine is the peripheral layer of exine. It is sculpted according to taxon-specific manner. The term sexine is derived “from ‘s’ in sculptured, and exine”. This layer has two well-developed subdivisions-ectosexine (also spelt as ektosexine) and endosexine.
Ectosexine (Fig. 4.6A-H) forms a roof-like layer on the periphery of a pollen grain. This layer covers a pollen grain partially or wholly. This layer is termed as tectum (n., pi. tecta, adj. tectate; Syn. tegillum).
Tectum may be absent when a pollen grain is described as intectate. When tectum is present a pollen grain is said to be tectate where the tectum appears to be as a continuous sheet without any perforations or covers 80 percent or more of the total surface of a pollen grain (Fig. 4.4).
In a semitectate or subtectate pollen grain the tectum is discontinuous where perforations exist. When the perforation is more or less one micron in diameter, it is termed as punctum (pi. puncta). Each punctum is like a cylinder and occurs as perpendicular channel on the tectum (Fig. 4.6D).
It has two openings — one on the supratectal’ (L. supra means over) and the other on the infratectal (L. infra means under) side. In many pollen grains the perforations on tectum are very large, more or less angular and regularly or irregularly polygonal. Such honeycomb-like perforations are described as reticulum (adj. reticulate).
Each reticulum has a roofless space termed lumen (pi. lumina) surrounded by sexinous ridges termed muri (sing, murus). Muri are the walls of a lumen and they anastomose to form reticulum. The murus stands on nexine, which is also the floor of lumen. So a reticulum has no tegillum (Fig. 4.6E & F).
Tegillum is present on negative reticulum. Negative reticulum is formed by grooves. Grooves on sexine anastomose and enclose sexine isles. A network of anastomosing grooves separates the sexine isles thus constituting an inverse or negative reticulum (Fig. 4.6G & H). Apart from punctum and reticulum small lumen occurs on tectum. Each lumen has very wide muri.
This type of perforation cannot be regarded as reticulum. Such perforations are referred to as scrobiculi (sing, scrobiculus). The lumina of reticula when become very small and the spaces that separate the lumina become wider, and then the lumina are called scrobiculi. The sculpture on the supratectal region of ectosexine is discussed later under surface ornamentation.
Endosexine consists of columella, baculum and void spaces separating the bacula from each other. The columella (pi. columellae, adj. columellate) is a column like element of sexine that supports the tectum at the top. A baculum is also a sexine element, which is slim, cylindrical rod like structure and stands on nexine. Each baculum is more than one micron in length and less than this in diameter.
Bacula are straight and often end abruptly. Erdtman (1969) illustrates different types of baculum and they are represented in Fig. 4.7. Erdtman (1969) regards two layers in sexine. The peripheral layer is the tectum and the inner is the bacules. In Fig. 4.8 the supratectal processes, the intratectal and infratectal details are illustrated.
Nexine occurs below the sexine and it is nonsculptured part of exine. The term nexine is derived “from ‘n’ in non-sculptured and exine”. Erdtman (1952) distinguished two subdivisions (ectonexine and endonexine) in this layer on the basis of thickness ‘and refractive zone’.
Ectonexine is the peripheral layer of nexine, exhibits more or less thick and ‘not very refractive zone’. Endonexine is the inner layer of nexine, more or less thin and exhibits ‘more refractive zone’. Erdtman (1948) reported the presence of a layer in between ectonexine and endonexine— termed mesonexine (Figs. 4.1A & 4.3).
Nexine seems to be homogeneous. In acetolysed pollen grains (ex. species of Malvaceae and Onagraceae) it is reddish brown while the sexine is yellowish. Erdtman (1969) differentiated two layers or subdivisions in nexine based on staining characteristics. The layers are nexine 1 and nexine 2.
In acetolysed pollen grains nexine 1 stains red with alcoholic fuchsin and nexine 2 does not show any staining characteristic. Nexine 1 is also called pedium (the Latin word pedium means foot layer) as the bacules stand on this layer. Sexine also stains red with alcoholic fuchsin like nexine 1.
Due to this similar staining characteristic Erdtman (1969) called sexine and nexine 1 together as exine 1. Nexine 2 is referred to as exine 2 (Figs. 4.1D & 4.8). Exine 1 and exine 2 are also referred to as ectexine and endexine respectively. Ectexine and endexine are distinguished on the basis of staining characteristics with basic fuchsin. The former stains positively with basic fuchsin while the latter remains relatively unstained.
Intine (endosporium, L. intinium):
Intine is the innermost layer of sporoderm. It is soft (malacodermic), fragile and pliable. It is colourless layer, encircles completely the protoplast and remains tightly attached to the plasma membrane. It is slightly resistant layer. It decomposes when boiled with acids and alkali. It is not well preserved in fossil spores and pollen. The thickness of intine is related to that of exine.
In the place where the exine gets thinner, the layer of intine becomes thicker. So thickened portions of intine occur under pores. Exceptions are observed in Carina, Musa, and Crocus etc. where a thick layer of intine envelops the protoplast uniformly. During germination of a pollen grain the intine emerges through the apertures forming the membrane of pollen tube.
Rowley (1959) recognized a layer in between exine and intine and termed it as mesine. This layer is faintly lamellate, acetolysis resistant and electron dense. Saad (1963) called this layer as medine.
Sporoderm stratification in a typical angiosperm pollen grain. N: neine, N1: nexine-1, N2: nexine-2 (syn. Exine-2), T: tectum, B: bacules, E2: exine-2 (syn. Nexine-2), P: opening on infratectal side of tectal perforation. [Tryphine: It is the deposition of structural materials (derived from break down products of tapetum) on the exine and contains the membranous components like mitochondria, endoplasmic reticulum etc. Pollenkitt, the other break down product of tapetum, differes from tryphine is lacking the membranous components. It is sticky and cements pollen togather during entomophilous dispersal.]
In this context it is worthy to mention the concept of Faegri (1975) regarding sporoderm stratification and its difference from that of Erdtman that are illustrated in Fig. 4.5A-D.