Lecture 03

Palynology

The study of the different features of pollen grains is known as Palynology, and it is useful for the identification and classification of plants, as well as in geological research, medicine and criminology.

The process of liberation of pollen grains from the anther is known as Anthesis, while the union of nuclei from pollen grains and ovules is called Fertilization, the mechanism of transference of pollen grains from the anther to the stigma is known as Pollination.

Classification of pollination

   A. Pollination takes place internally when

the flowers are closed…………………………..……. Cleistogamy

AA. Pollination takes place when

the flowers are open…………………………..……. Chasmogamy

  B. Deposition of pollen on the stigma of

the same flower…………………………………..Autogamy

                                                                                                              (Self pollination)

  C. Pollen deposited directly from the

anthers on to the receptive stigma…………….. Direct

CC. Pollen deposited on the stigma

by an external agent…………..................….. Indirect

BB. Deposition of pollen on the stigma of

another flower……………………………..………Allogamy

                                                                            (Cross pollination)

 D. Pollination by another flower of the               

same inflorescence on the same plant………………………….…..…… Geitonogamy

DD. Pollination by pollen from another

plant………………………………...……. Xenogamy

Types of Pollination Based on Agents of Pollination

A. Anemophyly: Pollination by wind.

B. Hydrophily: Pollination by the agency of water.

C. Zoophily: Pollination by the agency of the animals.

          1. Ornithophily: Pollination by birds.

          2. Chiropteriphily: Pollination by bats.

          3. Melacophily: Pollination by slugs and snails.

          4. Entomophily: Pollination by insects.

Pollen morphology

Although pollen grains are one of the smallest parts of the body of a flowering plant, they provide one of the richest sources of variation in morphological and structural features of the Plant. This variation concerns the following aspects:

A. Composition of pollen grains

The following forms of pollen may be recognized in flowering plants:

1.     Monads: Each pollen grain is independent from all others in the same pollen chamber at the time of anthesis.  This is the most common type of pollen composition in flowering plants.

2.     Dyads: The 4 cells resulting from the meiotic division of one pollen mother cell are separated into two bicellular masses of pollen grains, known as dyads.  This type is very rare.

3.     Tetrads: All 4 cells resulting from the division of one pmc, remain adhering to each other to form a tetrad of pollen grains.  The 4 cells are arranged in various spacial configurations so that the following types of tetrads can be easily distinguished:

a. Linear: All 4 cells are in one straight line,

b. T-shaped: in which two of the 4 cells form a line which is perpendicular to that formed by the other two.

c. Square: all 4 cells are in the same plane and form a square in polar view,

d. Tetrahedral: they are not in the same plane, only 3 of the 4 pollen grains can be seen from any direction, while only the shadow of the forth pollen grain may be observed.

4. Polyads:  The pollen grains of the same pollen chamber are grouped together into masses of definite shape, nurnber and arrangement known as polyads.  The number of pollen grains forming a polyad is constant for the species and may be 8, 12, 16, 24, 32 or rarely 64.

 5. Pollinia: All pollen grains in the some pollen chamber are united together into one mass known as the pollinium. This type is found only in two families of flowering plants; the Asclepiadaceae and the Orchidaceae.

B. Sporoderm stratification

The wall of the pollen grain (sporoderm) consists of two main layers: an inner soft layer called the intine and an outer hard-layer called the exine. The intine is thin and elastic, while the exine is relatively much thicker and extremely hard. Furthermore. while the intine is homogeneous in structure the exine is heterogeneous and consists of more than one layer.

The exine is made up of two main layers: an inner homogeneous layer called endexine (nexine), and an outer layer of numerous columellae ending in large globular bodies and known as the ectexine (sexine). Some palynologists regard the nexine as a distinct third layer of the sporoderm and call it the medine.  The globular bodies of the sexine may fuse together forming the outermost layer of the sporoderm and called the tegillum (tectum).  The outer surface of the tectum may be smooth or may be provided with some deposits of a characteristic patter. The relative thickness of the different layers of the sporoderm vary considerably from species to species.  The exine is usually much thinner in hydrophytes (water plants) than in xerophytes (desert plants).

C. Exine sculpture

The following are some of the types of exine sculpture common in angiosperms:

(a) Smooth: where the outer surface of the exine has no ornamentation. 

(b) Granulose: The exine surface is furnished with minute and sparse granules. For example, the pollen grains of the Gramineae and Liliaceae

(c) Rugulose or worty: The exine surface is provided with coarse and dense granules or worts which leave very little of the exine surface area uncovered.

(d) Reticulate: The outer surface of the exine may be covered by a reticulum (or network) of the same substance of the tectum.

(f) Echinate or spiny: The exins is provided with a number of sharp spines. The base of the spine may be solid or may consist of several columellae which carry the rest of the spine. The outer surface of the spine may be smooth or may carry worts of various shapes.

D. Number, Position and Character of aperture

NPC- System

`The pollen grain germinates by the production of some germination tubes which penetrate the sporoderm through certain holes called the germination apertures.  The number, shape and distribution of these apertures varies from species to species.  Rarely, the pollen grains-have thin exines and germination takes place through the rupturing of the exine. This is found mostly in hydrophytic plants. When the sporoderm has no obvious apertures the pollen grain is described as 'Atreme' or inaperturate'.

A special system has been devised to describe the cumber, position and character of the germination apertures in pollen grains.  It is generally known as the NPC-system.  It may be summarized in the following;

1. Number: Pollen grains with 1, 2. 3, A, 5, 6 and many apertures are called mono-, bi". tri-, tetra-, penta-.' hexa- and poly-aperturate respectively.

2. Positions: There are 3 different positions of apertures:

(a) polar: when the pollen grain has only one aperture situated at its pole, or two apertures placed at the two opposite poles of the pollen

 (b) Equatorial or zono-aperturate: where the grain has three or more apertures distributed at equal distances around its equatorial line.  Sometimes the apertures are arranged in two rows surrounding the equatorial line of the grain which is then called dizono-aperturate.

(c) Global or panto-aperturate:  in which the apertures are scattered all over the pollen wall at equal or unequal distances.

(3) Character:  According to the shape of the apertures in surface view, they can be distinguished into the three following types:

(a) Pores: where the aperture is more or less circular in surface view.

(b) colpi: these are fusiform or elongate apertures.

(c) colporate: which is a combination of the two previous types. The colpus is found in the outer layers of the exine while the pore in the inner layers, Both the pore and the colpus are concentric.

The pores and colpi are sometimes called simple apertures, while the colporate type is called compound apertures.

In order to describe the apertures of a pollen grain, one term is employed to cover their number, position and character.

For example, trizonocolpate means that the pollen grain has 3 fusiform apertures distributed at equal distances around the equatorial line. Polypantoporate groins have numerous pores scattered all over its exine, etc.