Why do flowers have different size and shape pollen grains?

What makes pollen grains special?

Why do flowers have different size and shape pollen grains?
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.





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
Reply:Flower Structure


The flower is the reproductive structure for flowering plants (angiosperms). Flowers are extremely diverse in size, shape, color, and so on. This makes them excellent tools for distinguishing plants. As a physiologist, I can identify a few plants from just leaf and stem, but most of the time I need a flower to identify a plant.





The flower is a short branch (stem with leaves). The nodes of this branch are very close together; the internodes are typically extremely short. The leaves of this branch are of four types: sepals, petals, stamens, and carpels. The short branch is called the receptacle and the four kinds of leaves are attached to this receptacle. In most flowers there are more than two leaves of each kind on the flower, so the leaves are in a whorled arrangement (more than two leaves per node). We'll consider the flower from bottom to top:





The lowest whorl on the receptacle is called the calyx. It is composed of a few or many sepals. In some species, sepals are green and photosynthetic. In other species, they are showy and almost indistingishable from petals.





For the detaisl.http://www.biologie.uni-hamburg.de/b-onl...
Reply:Uh, a shorter answer would be to avoid cross-pollination. Plants are all over the place, and most pollinators aren't specific to one species. So, bees and things usually have more than one kind of pollen stuck to them as they visit flowers. Only the pollen of the correct shape will pollinate a particular flower. That way you don't get rose pollen fertilizing say, a marigold.
Reply:Yes. This is what enables scientists to say with accuracy whether an animal lived and died in an area where a certain plant or climate existed (Mastodons ). Microfossils are another emerging technology that uses pollens identification. Fungal spores are also characteristic of their family, genus as well.