Symmetry

Pollen or spores may be symmetric or asymmetric. Asymmetric grains have no planes of symmetry and are either fixiform (with fixed shape, which is the common case) or nonfixiform (without fixed shape, very rare). Symmetric grains may be of two types: radiosymmetric (radial) grains have more than two vertical planes of symmetry, or, if provided with but two such planes, always with equilong equatorial axes. Bilateral spores are more or less flattened having two vertical planes of symmetry but in contradiction to the radiosymmetric spores with two such planes, the equatorial axes are not equilong. However, sometimes it is difficult to determine the symmetry as for example in Crypteronia and Isoglossa. Most Dicotyledons are radially symmetrical, whereas most Monocotydons and primitive Dicotyledons are

Symmetry Pollen Grain
Fig. 4.1 Types of pollen associations at the time of their release from anthers.

A. Tetrahedral B. Tetragonal

Fig. 4.2 Modes of pollen association in tetrads.

Crypteroniaceae

Fig. 4.3 Different types of pollen tetrads.

bilateral. Bilateral spores are also commonly produced in some Pteridophytes.

Size and Shape

Size is important since structural differences are sometimes inadequate for distinguishing species, and size becomes a reliable criterion. In Picea for example, such measurements have aided species identification. Pollen grains of angiosperms range from 5-200 ^m in diameter. However, most grains seem to fall in a range of 25 ^m-100 ^m in living angiosperms.

Very few pollen grains have 5 ^m as maximum diam. Families such as Boraginaceae (Myosotis), Piperaceae, Crypteroniaceae Cunoniaceae have

Fig. 4.4 Part of pollen showing distal pole and distal face, i.e., face away from the centre of the tetrad.
Proximal And Distal Pole Tetrad

Fig. 4.5 Pollen and spore showing polar axis (p), equatorial axis (e), distal pole (dp) and proximal pole (pp).

Fig. 4.6 Heteropolar: Pollen in which the distal and proximal faces of the exine are different either in shape, ornamentation or apertural system as in Echium vulgare (Boraginaceae).

pollen of lowest size ranges. However, pollen grains in excess of 200 ^m in diam. are recorded in certain species of Dispsaceaeae, Nyctaginaceae, Oenotheraceae; Onagraceae Malvaceae, Cucurbitaceae. Probably the largest (diam. up to 350 ^m) pollen grains are those of Cymbopetalum odoratissimum, a member of Annonaceae, (Walker 1971). Most of the studied monocots according to Cranwell have grains between 15-80 ^m. In marine angiosperms, Zostera marina has tubular pollen exceeding 2,500 ^m in length and 3-4 ^m in diameter. Amphibolis has tubular pollen exceeding up to 5 mm long (Ducker et al., 1978).

There often appears to be a direct relation between size and number of pollen produced per anther. For example, in Mirabilis only 32 very large pollen occur per loculus, whereas as many as 50,000 pollen grains are produced per loculus in Borago. In Rumex acetosa 30,000 grains are produced per stamen. It also appears that, as a rule the largest grains are produced by ephemeral flowers lasting only a day. Generally pollen of anemophilous plants are 'small' and those of entomophilous plants are 'large.'

Spores of certain Pteridophytes are extremely large, e.g., Selaginella exaltata megaspores have a diameter of about 1.5 mm. In Carboniferous deposits, megaspores measuring up to about 6-7 mm have been encountered as in Triletes giganteus.

SHAPE OF THE POLLEN GRAINS

The shape of pollen is often an important pollen morphological character. The shape of pollen varies in different views. The outline in polar view or Amb (L. Ambitus: short form Amb) is circular, triangular, square, pentagonal, rounded, three-lobed or in other geometrical shapes. Pollen and spores also differ considerably in their contours, walls (Figs. 4.7 and 4.8) and apices (Fig. 4.9). As early as 1943, Erdtman suggested certain terms to describe shapes of pollen grains based on the ratio of polar axis to equatorial axis P : E. In the equatorial view the ratio between the polar and equatorial diameters multiplied by 100 gives an indication of the shape. The following terms are used to describe the shape of the pollen grains. Here P refers to polar diameter and E refers to equatorial diameter.

CIRCULAR

TRIANGULAR

Fig. 4.7 Modes of contours of pollen walls in polar view.

CIRCULAR

TRIANGULAR

ELLIPSOIDAL

Fig. 4.7 Modes of contours of pollen walls in polar view.

STRAIGHT

CONVEX

CONCAVE

STRAIGHT

CONVEX

CONCAVE

Fig. 4.8 Modes of pollen side walls in polar view.

Fig. 4.8 Modes of pollen side walls in polar view.

ANGULAR

ROUNDED Fig. 4.9 Types of pollen apices in polar view.

ANGULAR

ROUNDED Fig. 4.9 Types of pollen apices in polar view.

TRUNCATED

d. Prolate spheroidal - P/E X 100 = 101 - 114 Subprolate- P/E X 100 = 115 - 133

SIZE CLASSES

Walker and Doyle (1975) have simplified the following six classes of size of pollen grains based on diameter or length of the longest axis.

In bilateral grains, pollen are plano - convex, concavo - convex, or biconvex in lateral view. In other views, different shapes such as ellipsoidal, lenticular, oval or other types are often present.

Fig. 4.10 A-F- Various shapes of pollen based on P/E x 100, A. Peroblate, B. Oblate, C. Suboblate, D. Prolate spheroidal, E. Prolate, F. Perprolate.

THE POLLEN WALL

The pollen grain wall is one of the most remarkable structures in plants. Each species of flower produces pollen with a uniquely structured wall. This uniqueness of the wall enables the identification of the parent plant when viewed under the microscope. When viewed optically with the naked eye, a mass of a specific pollen type may show a distinct colour, for example some members of the Asteraceae have orange pollen whilst some Onagraceae have grey pollen when seen en masse. Most pollen however are shades of yellow. This colour feature is of significance in honey analysis, melissopalynology (see Chapter 12). Microscopically, the wall may appear, smooth, spiny, furrowed, patterned, pitted, etc. Furrows (colpi) and pores may occur separately or in combination of various numbers. A complex terminology has been developed to describe wall ornamentation, furrows and pores.

The pollen grain wall (sporoderm) comprises two layers, the outer (sculptured) layer is called exine and an inner layer is the intine. The exine structure, described above is composed of a complex of substances collectively known as sporopollenin. All structures evolve in relation to function and persist as refinements by the process of natural selection. The pollen wall can be studied directly—ontogeny. By means of experimentation its function(s) may be determined. The evolutionary development of the pollen wall however can only be deduced hypothetically - phylogeny. The ontogeny of the wall has been touched on above. There are several functions attributable to the pollen wall: The physical contents - cytoplasm within -are adequately protected as a living, viable entity capable of germination under appropriate conditions; the pores provide means of water transfer and substances involved in pollen dispersal and pollen stigma (female receptive part of a flower) interactions are contained within. Sporopollenin is synthesized by the tapetum (Blackmore and Ferguson, 1986) and the exine prior to the completion of the synthesis and deposition of sporopollenin, serves as a communication surface for the selection and transport of supplies up to the germination stage described sporopollenin as extraordinary molecular complex. Sporopollenin protects the living internal contents from the harmful effects of the environment, for example, radiation hazards, due to its ability to absorb UV radiation. The lipid component acts as a seal being impermeable to water thereby protecting the grain against dessication (Thanikaimani 1986). Sporopollenin appears to be consistent in its properties throughout all taxa.

Pollen grain ontogeny in both gymnosperms and angiosperms have features in common; the initiation of the pollen mother cells from sporogenous central tissue; the ultimate meiotic divisions to produce a tetrads of haploid cells and the development of the sporopollenous exine wall prior to the development of the cell wall (intine).

POLLEN WALL

The pollen wall is often compared to mammalian skin and hence it is at times known as sporoderm, its main function being protection against desiccation and mechanical injury. The pollen wall in angiosperms and gymnosperms is most durable and resistant due to its chemical constituent in the form of sporopollenin. It is a complex chemical substance, which withstands physical and chemical reactions including strong acids.

DEVELOPMENT OF THE POLLEN GRAIN WALL

As early as 1911, Rudolph Beer noted the sculpturing elements present on pollen while they were still within the pollen mother cell wall. The timing of the appearance of the sculpturing elements related to the appearance of fibrils that extended from the nucleus to the cell wall while the sculpture formed. Recent studies have elucidated the following stages of pollen grain wall formation.

As soon as the cells of the tetrad are defined and separated by a callose wall, a cellulose layer forms between the plasmalemma of the pollen and the cellose. The cellulose completely surrounds the pollen grain wall except where apertures develop. At the site of apertures, an area of endoplasmic reticulumn moves to the plasmalemma, which in turn lies against the callose tetrad wall. The association of the endoplasmic reticulumn with the aperture area prevents cellulose from forming over areas with which it is associated. The apertures are formed at the three areas where each grain comes in contact with the other grains in the tetrad. In the cellulose layer, probacula or procolumellae form, which are composed of lipoprotein. Thus, before sporopollenin begins to be deposited, the aperturation and columellae patterns are established.

After the probacula form, extensions of their bases extend and connect the probacula to form the Nexine 1 (foot layer). At this time sporopollenin is deposited in the probacula. This is called the primexine stage. Once the pollen grain is released from the tetrad, the pollen grain expands and the primexine stretches and becomes thinner. As the primexine stretches more sporopollenin is deposited. It is at this stage that the tectum and foot layer become developed (Fig. 4.11).

The nexine 2 (endexine) and the cellulose intine begins slightly before the separation of pollen from the tetrad. Nexine 2 is built up as sporopollenin lamellae are produced at the plasma membrane. The origins of sporopollenin differ in different parts of the pollen wall. The primexine and nexine 2-sporopollenin probably comes inside the haploid spore. After

Sporopollenin Composition

Sculpture

^ Tectum

— Columellae

— Foot layer and endexine v Intine

Pollen grain Aperture

Fig. 4.11 Pollen wall stratification.

Sculpture

^ Tectum

— Columellae

— Foot layer and endexine v Intine

Pollen grain Aperture

Fig. 4.11 Pollen wall stratification.

the pollen tetrad has divided, when the callose wall breaks up the source of sporopollenin is thought to be with the tapetum, which surrounds the pollen sac. Within the cells of the tapetum, one finds Ubish bodies, which are believed to be carotenoid precursors of sporopollenin. As the tapetum disintegrates, lipids and proteins that form the pollenkitt are released.

CHEMICAL COMPOSITION OF POLLEN WALL

It has been experimentally proved that the exine is exceedingly hard and resistant. It can be treated by strong acids or bases without being destroyed, or heated up to almost +300°C .

Oxidation processes and certain biological organisms (Phycomycetes, Actinomycetes and Bacteria) may corrode and destroy the exine. Pollen and spores originally embedded in sediment or peat, maintain their structure ±intact throughout the geological ages (from Palaeozoic and onwards). It is still possible to study their morphology and use them to date various geological strata. Chemically seen the exine is composed of sporopollenin, a substance resembling lignin and formed through polymerization of hydrocarbons, carotenoids and/or carotenoid esters (pine pollen: C90 H158 O44). Certain recent green algae, e.g. Chlorella, and fossil ones from Devonian appear coated with sporopollenin substances. In ferns an extra-exinous layer (perine) often occurs. The spore wall (including the outer layer, perine) of fern and moss-spores resembles that of pollen as to resistance, whilst that of fungal spores is chemically different.

POLLEN WALL STRATIFICATION

Basically the pollen wall is divided into two layers. The inner layer is called as intine and the outer layer is referred as exine (Fig. 4.12). The intine though less resistant to acids is an essential layer composed of cellulose and pectins. It forms the wall of the growing pollen tube and storage functions. Exine is more resistant, hard, sclerodermatous and highly variable layer in its structural characters. Functionally it is a nonessential layer, but most important for pollen morphology on account of its variability. Erdtman preferred to distinguish a pollen wall into the inner layer intine and outer layer as sclerine composed of the inner layer exine and the outer layer perine, which is thin, membranous, less resistant layer present prominently in some moss and pteridophyte spores. Perine is destroyed during acetolyses. Perine is absent in most of the angiospermous pollen and hence sclerine is actually synonymous with exine (Table 4.1a).

An important part of Sclerine is Sexine (S stands for Sculpture) which is actually Sculptured exine. It is further divided into inner Endexine and an outer ectosexine. The inner non-sculptured part of the exine is termed as

Pollen Wall Structure
Fig. 4.12 Generalized structure of pollen wall showing different layers.
Table 4.1a Sporoderm stratification, Erdtman 1952

P E R i N E

S

S C

SCULPTINE

P

E C T O S E X i N E

O

SEXINE

R

L

E

E N D O S E X i N E

E

X i

E R M

R

E C T O N E X i N E

N E

N E

NEXINE

E N D O N E X i N E

NEXiNE

iNTiNE

Sexine = from S in Sculptured exine Nexine = from N in Non-sculptured exine.

Sexine = from S in Sculptured exine Nexine = from N in Non-sculptured exine.

Nexine (N stands for non-sculptured part of exine). Nexine is divided into outer thick, not very refractive layer known as ectonexine, and inner; more refractive layer endonexine. Erdtman differentiated one more innermost layer of nexine thus classifying nexine into Nexine 1, Nexine 2 and Nexine 3 from inside to outside (Figs. 4.13 and 4.14).

Further classification of exine along with its morphological characteristic features is discussed under a separate pollen morphological category namely exine stratification. In a way pollen morphology is essentially exine morphology. Exine is often compared with fingerprints on account of its highly variable characters which are of great diagnostic value.

Pollen Wall Stratification

Tectate pollen wall

Fig. 4.13 Structure of tectate pollen wall: Pollen wall with tectum which is formed due to fusion of sculpture elements of sexine such as columellae and forming a roof over endexine.

Tectate pollen wall

Fig. 4.13 Structure of tectate pollen wall: Pollen wall with tectum which is formed due to fusion of sculpture elements of sexine such as columellae and forming a roof over endexine.

Tectate exine Pilate exine

Tectate exine Pilate exine

Pollen Wall

Tectate pollen wall Intectate pollen wall

Fig. 4.14 Exine stratification to show tectate and lntectate (pilate) types.

Tectate pollen wall Intectate pollen wall

Fig. 4.14 Exine stratification to show tectate and lntectate (pilate) types.

THE STRUCTURE OF EXINE OF POLLEN WALL

Among the flowering plants one of the basic elements of the sexine appears to consist of small drumstick shaped rods (pila), projecting at right angles from the endexine surface.

Each pilum has a head (caput) supported by a rod like baculum the capita forming the upper part of sexine (ektosexine) The bacula form the lower part of sexine (endosexine). If the capita amalgamate or hypothetically a layer of some sort should be formed on the top of pila, coalescing with or enveloping the capita while leaving the bacula free, a tegillum (tectum or small roof) is formed. The layer below the tectum is referred as Infratectum, which may be granular, columellar alveolar or structureless as shown in (Fig. 4.15).

Infratectum

Nexine 1 Nexine 2

Intine

Tectate pollen wall

Fig. 4.15 Tectate pollen wall showing details of lnfratectum: pl. Infratecta, adj. Infratectate, a general term for the layer beneath the tectum, which may be granular, columellar, alveolar or structureless.

Tectate pollen wall

Fig. 4.15 Tectate pollen wall showing details of lnfratectum: pl. Infratecta, adj. Infratectate, a general term for the layer beneath the tectum, which may be granular, columellar, alveolar or structureless.

The exine shows morphological features (comparable to fingerprints) that are of high diagnostic value. In simple pollen, the wall is composed of only two layers an inner uniformly structured endexine and a single layered outer ektexine that may have external sculpture or may be smooth.

The tectate (roofed) wall is made up of three or more layers: the endexine, columellae or granules fused at their apices to form a 'roof' , and sculptured elements deposited on this layer. In contrast, in some pollen grains the wall will be without tectum and is referred as intectate or atectate, wherein the sculpture elements such as baculae or columellae, etc., are free from each other without forming a roof over endexine (Fig. 4.16-B). In addition, several illustrations showing the structure of Tectate, Semitectate and Intectate pollen walls with suitable examples have been incorporated in Figs. 4.16-A-a to 4.16-A-o.

L - O ANALYSIS OF THE POLLEN WALL

It is common knowledge that the structure of a pollen wall under the microscope looks different if the adjustment of microscope focus, particularly the distance from the mounted pollen and objective lens of the microscope is changed. Under a different focus the same pollen wall feature exhibits aiiiio—,

Nexine Nexine 2 Intine

Intectate pollen wall

Fig. 4.16B Structure of Intectate pollen wall: pollen grain wall without a tectum. The sculpture elements of sexine are free from each other without forming a roof over endexine, e.g. Viscum (Loranthaceae), llex (Araliaceae).

QQQQQQQQQfl tectatee. g. Aconitum

QQQQQQQQQfl tectate e. g. Thelycrania

A-c intectate e. g. Popvlus qQQQQQQQQq tectate e. g. Nym.phaid.es

gflfotfe semitectate e. g. Polemonium A-e

OOODÛQÛÛÛû tectate e. g. Menyanthes A-f semitectate e. g. Saxífraga oppositifolia

nOOO 0 QQn Q tectate e. g. PLantago

semitectate e. g.Cyperaceae

SLWiZJB?

tectate e. g. Cerastium tectate e. g.Fagopyrum

AAA^

(\QQQQQQQ/7 tectate e. q. Malva

Fig. 4.16 A-a-o Structure of Tectate, Semitectate and Intectate pollen walls with suitable examples.

different patterns such as bright or dark. This analysis is referred as L - O analysis (adapted from H. Welcker 1885 for diatoms) where L stands for Lux or light or bright and O stands for obscuritas or darkness (Erdtman, 1956). In glycerine-jelly mounted pollen, elements projecting from exine are bright first and dark then, holes (perforations) appear dark first and then bright. The L-O analysis should preferably be done under oil immersion. By using this analysis routinely, the pollen wall morphology can be properly described. This can be explained with the help of the following example and illustration of L-O analysis of a sexine of pollen wall, which is described as punctitegillate, spiniferous.

A sexine like that shown in Fig. 4.16-c (E: a) exhibits different patterns at different adjustments of the microscope. At high adjustment, small white islands produced by the spinules are seen (E: b: 1). When focusing at a slightly lower level the same islands turn dark (E: b: 2-3). At medium adjustment very small dark islands appear, produced by the puncta in the tegillum (E: b: 4). They later become very bright (E: b: 5). At a lower adjustment numerous small white islands, caused by the baculae appear and later likewise turn dark (E: b: 6, 7). These patterns are referred to as S, T and I Patterns respectively (S: supra tegillar, T: tegillar, I: infra tegillar).

EXINE SCULPTURE

In the pollen grains most endexine layers are uniform in structure and are undifferentiated. The ektexine (sexine) usually is not uniform structurally; differentiation often develops in the form of irregularities on the exterior, which are known as sculpture patterns. Sections of the pollen wall illustrating the common types of sexine sculpture elements and sculpture

Pollen Grains Symmetry

Fig. 4.16c L-O analysis of pollen wall.

Fig. 4.16c L-O analysis of pollen wall.

patterns are depicted in Figs. 4.17-a-e and Figs. 4.18-a-l respectively. Combinations and intergradations of these forms are common.

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Responses

  • alfie
    How ornamentation of pollen grain is done?
    3 years ago
  • rosalia
    What is atectate pollen?
    2 years ago
  • diana
    What are the sizes. and shapes of pollen spores and grains?
    2 years ago
  • Ilmari
    What is sporoderm stratification?
    2 years ago
  • gianni
    Why pollen grains outer layer exhibit patterns?
    2 years ago
  • Biniam
    What is the tectum side of a pollen grain?
    1 year ago

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