Table of Contents:

1. Abstract
2. Introduction
3. Materials and Methods
4. Results

1.     Figure 1
2.     Figures 2 and 3
3.     Figure 4
4.     Figure 5
5.     Figure 6
6.     Figures 7 and 8
7.     Figures 9 and 10

5. Discussion
6. Notes
7. Bibliography

Abstract
Pollen morphology is closely related to its function. Many of the features present in pollen grains have helped the species of plants to which it belongs to adapt to life on land, be able to disperse its pollen, and fertilize the female eggs to produce new seeds that would give rise to new plants.  I have collected four samples of pollen from angiosperms and studied their detailed morphology.  This paper studies the correlations between the structure and function of pollen grains of those four species angiosperms.

Introduction

Pollen grains are structures that house the male gametophyte generation of angiosperms and gymnosperms (seed plants).  They are also the vehicles in which the male gamete genetic code is carried to the female gamete.  Pollen grains develop in the anther in angiosperms.  It travels and is deposited on the stigma of a receptive flower.  In gymnosperms, pollen develops in the male cone, travels, and fertilizes the ovules in the female cones to produce seeds.  Each pollen grain consists of the two celled male haploid plant enclosed in a thickened wall.

The casing that houses the male gametophyte has a very complex structure that is reflective of that specific species' functional adaptations.  The exine is the outer layer of a living pollen grain.  It is composed of sporopollenin, with small quantities of polysaccharides.  Sporopollenin is very chemically stable¹ and it is resistant to to almost all kinds of environmental damage.  It is equipped with apertures.  It it divided into two sub layers: the outermost sexine and the unsculptured underlying nexine.  The sexine has surfaces that are sculptured in elaborate ways, with reticulately arranged perforations.  These give the exine and amorphous or granular appearance.  The inner layer of a living pollen grain is called the intine.  It is composed of cellulose and is very similar in construction to ordinary plant cell walls.  A layer called the endexine separates the sexine and intine.  The endexine has a laminated appearance.

Pollen grains are generally classified according to their physical appearance.  There are three criteria of classification: 1) the number and position of the apertures; 2) the shape of the pollen grain as a whole; and 3) the fine elaborate structure on the sexine.  Apertures are any missing parts of the exine, which are independent of the exine pattern.  Apertures are big and they cut across the fine structure pattern on the surface of the pollen grain.  There are two types of apertures: pori or pores are mostly isodiametric apertures, although the can be slightly elongated with rounded ends; colpi or furrows are long and boat shaped with pointed ends.  Colpi are thought to be more primitive.  In living pollen grains these apertures are not actually open.  Instead, a very thin layer of exine covers them.  Grains with pori are called porate; those with culpi are called culpate; and those with both pori and culpi are called colporate.  If their apertures are arranged equidistantly around the equator of the pollen grains they are assigned the prefix zono-; if they are scattered all over the surface of the pollen grain they are assigned the prefix panto-.  The number of apertures is also indicated by prefixes: mono- for one aperture; di- for two apertures; tri- for three apertures; and so on.

The shape of a pollen grain refers to the shape of their outline in polar and equatorial views.  The shape of a grain can sometimes be useful in identifying of pollen species, but not usually. I may vary considerably within one grain type, and sometimes within one species.

The sculpture refers to the fine structure and pattern of the sexine.  It is composed of small radially directed rods.  If these rods support something (such as a plate or a small knob) they are called columnellae; if they do not support anything they are called bacula.  The shape of the rods can further classify them.  If they are club shaped they are called clavae; if they are sharply pointed they are called echinae; if they have swollen heads they are called pila; and if they are short and globular they are called gemmae.  There are many more classifications for the shape of rods on the surface of the sexine, but these four are the most common.

For this project, I have observed and photographed pollen grains from four different species of angiosperms.  These species are:

 

Schumbergera bridgesii, commonly called the "Christmas Cactus".  This plant can be found in the Succulent House of the Lyman Conservatory at the Smith College Botanical Gardens.  It is originally from Brazil.

Nusturtum2, commonly called the "Jewel of Africa".  This plant is native to South Africa (and is named after the city of Cape Town, "Jewel of Africa").  I can be found in the Show House of the Lyman Conservatory.

Osteospermum ecklonis, commonly called a "Silver Sparkler".  This plant it native to the coast areas of Western Cape, South Africa.  This plant is also on display at the Show House of the Lyman Conservatory.

Camellia japonica, commonly called a "Lady Vansittart" or "Japanese Camellia".  This plant is native to the mountain area on the west side region in Japan.  Many varieties of this plant can be found along the Camellia Corridor in the Lyman Conservatory.

Materials and Methods

In preparation for this project, I collected pollen specimens from four species of plants at the Lyman Plant House.  Pollen is considered a hard and wet specimen therefore it required of drying and coating for preparation to be observed and photographed using the Scanning Electron Microscope.  The pollen samples were mounted on 12 mm diameter support stubs.  I then let the pollen grains air dry for about four hours.  The excess pollen was removed by blowing on it with compressed gas duster.  I used a sputter coater³ for coating the specimen with a thin layer of metal.  This layer ensures an ample supply of secondary electrons for the signal⁴.  Specimens were scanned on the JEOL SEM at an accelerating voltage of 5 kV to dissipate charging and consequent distortion of the image.  I used a low working distance (between 8 mm and 15 mm) for finely detailed images; and the probe current was kept high (>12) for maximum resolution.

Results

Figure 1. Shumbergera bridgesii

Figures 2 and 3: Shumbergera bridgesii

Figure 4. Nusturtum

Figure 5. Nusturtum

Figure 6. Osteospermum ecklonis

Figures 7 and 8: Osteospermum ecklonis

Figures 9 and 10: Camellia japonica

Because all of the specimens used for this project are angiosperms, the surfaces of the sexine have very elaborate sculpturing.  Had we used some gymnosperms specimens, we should have found that their sexine were much less distinct.  Fortunately there is much fine detail on these images that I have used to draw some lines between the structure of pollen grains and its function.

I will divide this discussion into two main focuses:  the dispersal of pollen grains and the fertilization of the eggs in the ovum.  In terms of the dispersal of the pollen grains, there are two pollen features to take into account: the size and shape of the pollen grain and the fine structure of the sexine.  The size of pollen grains says a lot about its dispersal methods.  If a pollen grain is to be wind pollinated it has to be small enough so that gravity will not pull it down against a slight wind current.  In other words, wind dispersed pollen has to be small enough so that it can "fly".  It also has to be small enough so that once the pollen grain touches its pollinator gravity will not pull them off.  In other words wind dispersed pollen grains have to be small enough to be able to stick to surfaces that they come in contact with.  Large and spherical pollen grains, however, are good for long distance dispersal.

The fine structure of the sexine can also say a lot about the methods of dispersal of pollen grains.  The more elaborate the sculptures are, the more grip they provide for the attachment of the pollen grain to its pollinator.  So the smaller pollen grains that are wind dispersed have practically no sculpturing, excepting germinal apertures of the simplest kind or germinal furrows of the simplest kind. While those that are insect or bird pollinated are much more elaborate.  The sculpturing may provide for even more specific control over its method of pollination.  Some sculptures may be adapted for a specific species of pollinator, such as a particular type of bee, beetle, bird, or bat.  These exclusive relationships ensure that the pollen will not get wasted by being carried to the flower of a different species.  While other sculptures may be more general in their pollinator specificity.

All of the species used for this report are angiosperms and that suggests that they are all either bird or insect pollinated (since they have the flowers to attract such pollinators).  But notice that the magnifications and scale marker bars indicate that the pollen grains of the Nusturtum species are very small (between 20 and 25 mm).  Their surface does not have very elaborate sculpturing.  This suggests that the Nusturtum may be a species of angiosperms that relies heavily on wind dispersal of its pollen grains.  The echinae in the pollen grains of Schumbergera bridgesii and Osteospermum ecklonis seem to be well adapted to attach to the bodies of birds or insects.  They are also larger than the other specimens and this indicates that they are animal pollinated.

Another factor in determining the method of pollen dispersal of an angiosperm is to look at pollen number.  Plants that rely on wind dispersal of pollen grains produce pollen in much greater numbers, while those that are insect or bird pollinated produce pollen in lesser quantities.  Unfortunately, I did not quantify my samples of pollen, and therefore am unable to draw any conclusions based on that factor.  I did, however, use the same method for collecting the samples for all four species.  The amount of pollen on the support stub for the Japanese Camellia was much greater than that for the other three species.  There was less of the other three and they were much better dispersed throughout the flat surface of the support stub.  If I were to use this as reliable data, it would indicate that the Japanese Camellia also relies on wind dispersal of pollen grains.  Notice that the granulated surface of the sexine of this species of pollen and the size of this particular pollen grain, although long, it is very small in diameter if looked at its polar view.  This also supports the indication that it relies on wind dispersal of its pollen grains.

In terms of the ability of the pollen grain to fertilize the eggs in the female part of the angiosperm plant, I will mainly talk about apertures, although fine sculpturing may also play a part.  When a pollen grain comes to rest on the stigma of another flower, the moist secretion that it encounters there stimulates it to produce a slender pollen tube.  In order for this to happen the pollen has to recognize itself within the vicinity of an ovule.  Usually apertures provide the pollen grain with a medium for the recognition of protein signals so that it can develop a pollen tube that discharges sperm.  There may be special pores provided for the emergence of the pollen tube, especially if the outer coat of the grain is thick, or the exine may simply rupture.  The pollen tube penetrates the stigma and the style, finally reaching one of the ovules, where it discharges two male gametes and fertilization is effected.  This is why palynologists suggest that the more apertures and the bigger those apertures are, the greater the chance that that pollen grain will fertilize an egg and produce a seed.  Apertures such as the ones shown in Figures 1 through 3 (Shumbergera bridgesii) would provide the plant with greater chances for fertilization because they are found in many numbers.  The other three species have pollen grains with three apertures, while the Christmas Cactus has six.  Equally, the size of the apertures shown in Figures 6 through 8 (Osteospermum ecklonis) would also provide its plant with a greater chance for fertilization to occur.

In conclusion, the many forms of pollen grains, including variations in their shape, size, number and arrangement of apertures, and the fine sculpturing of their sexine, are adaptations to help the pollen grain better perform its function of fertilizing the female gametophytes and forming seeds that will give rise to new generations of plants.  The use of resistant, wind dispersed, or insect or bird dispersed pollen to bring together gametes is an adaptation that has led to the great success of angiosperms on land.

1. Moore, et al. Pollen Analysis.
2. The label on this plant at the Lyman Plant house only indicated the genus name "Nusturtum", but did not indicate the species name.  I did search for the genus name and the common name of the plant to try to find out the species name of the Jewel of Africa, but was able to find it.
3. The instructions for the sputter coater were followed exactly as the "Scanning Electron Microscopy and X-Ray Analysis" manual described them.
4. According to the "Scanning Electron Microscopy and X-Ray Analysis" manual.
5. This image was not taken with the intention of including it in this report.  It ended up being the only image that I had that showed the polar view shape of this pollen grain with any clarity, so I was forced to include it as proof for the arrangement of the apertures.  This explains the poor resolution of the image (high working distance, low probe current, low magnification).

1. Briggs, D, John Brady, and Bob Newton. Scanning Electron Microscopy and X-Ray Analysis.  (Northampton, MA: Smith College, 2000)
2. Campbell, Neil A., Jane Reece, and Lawrence Mitchell.  Biology.  (California: Benjamin/Cummings, 1999). 730-740.
3. Erdtman, G.  Handbook of Palynology: an introduction to the study of pollen grains and spores.  (New York: Hafner, 1969).
4. Faegri, Knut, and Josh Iversen.  Textbook of Pollen Analysis. (New York: Hafner, 1964).
5. Moore, P.D., J.A. Webb, and M.E. Collinson.  Pollen Analysis.  (Oxford: Blackwell, 1991).
6. Wodehouse, R.P.  Pollen Grains: their structure, identification, and significance in science and medicine.  (New York: McGraw-Hill, 1935).
7. "The Osteospermum Home Page" at http://www.osteospermum.fsnet.co.uk/index.html.