BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.5. Sexual Reproduction in Flowering Plants

21.5.5. Pollination

Once pollen grains have formed within the pollen sacs, the Cells forming the walls of the anther begin to dry out and shrink. This creates tension that eventually leads to the splitting (dehiscence) of the anthers along longitudinal slits formed on their lateral surfaces (Fig. 21.21), thereby releasing the pollen.

The transfer of pollen grains from an anther to a stigma is called pollination. (Take care not to confuse pollination with Fertilization.) Pollination is essential to enable the male Gametes developing within the pollen grains to meet the female gametes. Within the pollen grains, the male gametes are protected against desiccation. Over the course of evolution, mechanisms have developed that ensure successful pollination.

The transfer of pollen from the anther to the stigma of the same plant is termed self-pollination. The transfer of pollen from the anther of one plant to the stigma of another is termed cross-pollination.

Relative Advantages of Cross-Pollination and Self-Pollination

Cross-pollination leads to cross-fertilization and an increase in genetic variation. Thus, it acts as a form of outbreeding. Many plants possess special adaptations that promote cross-pollination, as described below. However, this process is accompanied by a wasteful expenditure (from the plant's perspective) of vast amounts of pollen.

The advantage of self-pollination, which leads to self-fertilization, lies in its greater reliability, especially when representatives of a given species are relatively rare and separated by great distances. This is because self-pollination does not depend on external factors such as wind or insects. Self-pollination is also advantageous in harsh climatic conditions where insects are scarce, such as high in the mountains. However, self-fertilization, being an extreme form of Inbreeding, can lead to reduced offspring vigor (see section 27.4.1). Examples of self-pollinating plants include groundsel and chickweed; their flowers produce no nectar and lack scent.

Both cross-pollination and self-pollination have their respective Advantages and disadvantages. Many plants have evolved adaptations that favor cross-pollination while at the same time allowing for self-pollination should cross-pollination fail for any reason. For example, in violets and wood sorrels, certain flower buds never open, making self-pollination inevitable.

Mechanisms Favoring Cross-Pollination

DIOECIOUS AND MONOECIOUS PLANTS. In dioecious species, male and female flowers are borne on separate plants, making self-pollination impossible. In monoecious species, separate male and female flowers occur on the same hermaphroditic plant. This also favors cross-pollination, though self-pollination remains possible.

21.2. Dioecious plants are relatively rare, despite the advantages of cross-pollination. Suggest two possible reasons for this.

21.3. Dioecy (unisexuality) is widespread among animals.

Consider why this system proved more suitable for animals than for flowering plants?

PROTANDRY AND PROTOGYNY. The maturation of anthers and stigmas sometimes occurs at different times. When anthers mature first, the condition is termed protandry, whereas if the stigmas mature first, it is termed protogyny. Protandry is much more common; examples include deadnettle, dandelion, and sage (Fig. 21.26). Protogyny is characteristic of squill and figwort. In most cases of both protandry and protogyny, There is a period when both anthers and stigmas are mature, making self-pollination possible if cross-pollination has not occurred.

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Fig. 21.26. A bee entering a meadow sage flower. The bee's HEAD presses against the lever mechanism to which the stamens are attached, causing them to swing down and deposit pollen onto the bee's abdomen. As the flower ages, its stigma elongates. If the bee subsequently visits an older flower, its abdomen may brush against the stigma, transferring the pollen adhering to its abdomen onto the stigma. As a result of these events, cross-pollination takes place.

SELF-INCOMPATIBILITY (SELF-STERILITY). Even if self-pollination occurs, pollen grains often fail to develop or grow very slowly, which prevents self-fertilization or reduces its probability. In all such cases, a specific inhibition of pollen tube growth down the style occurs, governed by self-incompatibility genes.

In cases of self-incompatibility, cross-pollinations can sometimes also be incompatible. The most efficient utilization of pollen occurs when a high proportion of crosses are compatible. An extreme example is clover, in which all plants are self-incompatible, yet cross-incompatibility occurs in fewer than 1 in 22,000 pairs.

21.4. Self-incompatibility is controlled by multiple alleles.

Assuming that

a) there are three alleles — S₁, S₂, S₃, and

b) self-incompatibility arises when the pollen grain and the stigma cells share a common allele,

then what proportion of pollen grains from a plant with the genotype S₁S₂ will be capable of successfully germinating on a plant with the genotype S₂S₃?

SPECIAL STRUCTURAL ADAPTATIONS. Most hermaphrodite flowers possess structural features that promote cross-pollination. In insect-pollinated flowers, stigmas typically project above the anthers, preventing pollen grains from falling directly onto the stigma of the same flower. When an insect carrying pollen from another plant visits such a flower, it touches the stigma first. Then, as the insect searches for nectar, it either becomes dusted with pollen or brushes it onto itself. This is the case, for example, in dead-nettle pollination (Fig. 21.19). A more primitive mechanism involves the stigma touching the insect as it lands on the flower, as seen in sweet pea pollination (Fig. 21.18). Such mechanisms are usually combined with protandry and protogyny, and the flowers are often complex and zygomorphic, as in the dead-nettle.

Flowers attract insects by providing food (nectar or pollen) and stimulating their Vision and SENSE OF SMELL. This is made possible by specific traits of the flowers, which are discussed below.

In many wind-pollinated flowers, the stamens, the flower as a whole, or the entire inflorescence hang downwards, so that the released pollen lands on the plant before being carried away (as, for example, in hazel).

21.5. Fig. 21.27 shows Two Types of primrose flowers found in nature in roughly equal numbers, differing in style length (heterostyly) and stamen arrangement.

a) It is known that bees suck nectar from the lower part of the corolla tube; explain why cross-pollination occurs mainly between long-styled and short-styled flowers rather than between flowers of the same type?

б) What is the advantage of such a system?

Although the variations in style length noted in question 21.5 and in Fig. 21.27 promote outbreeding, a far more effective mechanism in this regard is self-incompatibility existing between long- and short-styled primroses, whereby cross-fertilization occurs only between flowers of different types. The genes controlling incompatibility, style length, and stamen height are located very close together on the same chromosome and behave as a single genetic unit.

Fig. 21.27. Structural differences in primrose flowers. A. Long-styled flower. B. Short-styled flower.

Wind Pollination and Insect Pollination

Pollen grains must be transferred to the female reproductive structures of flowers. In the Cytology/cytology/16.html">Early stages of evolution, pollen was distributed by the wind (anemophily), but this transfer mechanism is highly inefficient, as pollen delivery to flowers depends entirely on chance. Many flowering plants, such as oak, hazel, or grasses, are still wind-pollinated, but they must produce enormous quantities of pollen, which requires a large expenditure of substances and energy. The Characteristic Features of wind- and insect-pollinated flowers are listed in Table 21.3.

Table 21.3. Typical differences between wind-pollinated and insect-pollinated plants

Insects specialized for feeding on flowers appeared in the fossil record simultaneously with flowering plants; these include bees, bumblebees, wasps, moths, and butterflies. This suggests that insect pollination arose shortly after the appearance of flowers. An insect is a much more reliable pollen vector than the wind. It is capable of transferring a small amount of pollen from the anther of one plant to the stigma of another. As a result, a special relationship evolved between flowers and insects. The reward that the insect receives from the flowers is food in the form of nectar or pollen. In some cases, the insect and its pollinated plant are so interdependent that neither can survive without the other, as, for example, the yucca and the yucca moth.

Insect pollination has another important advantage: it favors cross-pollination and thus cross-fertilization; therefore, the flower modifications described below that facilitate insect pollination can be added to the list of traits promoting cross-pollination.

In entomophilous plants, flowers are usually large, with brightly colored or white petals to attract insects; if the flowers are small, they are gathered into inflorescences. The petals often feature stripes, spots, or more intensely colored areas that guide insects to the nectaries; these are found, for example, in violets, pansies, orchids, and foxgloves. Insects can perceive ultraviolet rays, which are invisible to humans, so flowers that appear white to us may be perceived by insects as colored. More specific than coloration are the scents emitted by flowers; some of these, such as lavender and rose, are used in the perfume industry. Sometimes flowers possess the scent of decaying meat, which attracts carrion-feeding insects, or the smell of manure, which attracts dung flies. The shape of the flower can also serve as a specific recognition cue.

One of the most complex and bizarre mechanisms for ensuring cross-pollination is found in orchids: the flowers of some orchids resemble female wasps in shape, color, and scent, the resemblance being so convincing that males attempt to copulate with these flowers (Fig. 21.28). During these attempts, the insect brushes pollen onto the flower, and upon leaving, carries away its pollen to be transferred to another flower.

An example of a wind-pollinated flower is the grass meadow fescue, shown in Fig. 21.20. Among insect-pollinated flowers, we can mention the buttercup (Fig. 21.17), sweet pea (Fig. 21.18), and white dead-nettle (Fig. 21.19).

Fig. 21.28. A male digger wasp attempting to copulate with a flower of the bee orchid, which it has mistaken for a female wasp.