for crossing, and subsequently to produce seed.
5.4.3 The flower structure
Genetic manipulation of flowering plants by conventional tools is accomplished through the technique of crossing, which involves flowers. To be successful, the plant breeder should be familiar with the flower structure, regarding the parts and their arrangement. Flower structure affects the way flowers are emasculated (prepared for crossing by removing the male parts to make the flower female). The size of the flower affects the kinds of tools and techniques that can be used for crossing.
5.4.4 General reproductive morphology
Four major parts of a flower are generally recognized – petal, sepal, stamen, and pistil. These form the basis of flower variation. Flowers vary in the color, size, numbers, and arrangement of these parts. Typically, a flower has a receptacle to which these parts are attached (Figure 5.3). The male parts of the flower, the stamens, comprise a stalk called a filament, to which is attached a structure consisting of four pollen‐containing chambers that are fused together (anther). The stamens are collectively called the androecium. The center of the flower is occupied by a pistil, which consists of the style, stigma, and ovary (contains carpels). The pistil is also called the gynoecium. Sepals are often leaf‐like structures that enclose the flower in its bud stage. Collectively, sepals are called the calyx. The showiest parts of the flower are the petals, collectively called the corolla.
Figure 5.3 The typical flower has four basic parts: petals, sepals, pistil, and stamen. The shape, size, color, and other aspects of these floral parts differ widely among species.
5.4.5 Types of flowers
When a flower has all the four major parts, it is said to be a complete flower (e.g. soybean, tomato, cotton, tobacco). However, if a flower lacks certain parts (often petals or sepals) as is the case in many grasses (e.g. rice, corn, wheat), it is said to be an incomplete flower. Some flowers either have only stamens or a pistil, but not both. When both stamens and a pistil occur in the same flower, the flower is said to be a perfect flower (bisexual), as in wheat, tomato, and soybean. Some flowers are unisexual (either stamens or pistil may be absent) and are called imperfect flowers. If imperfect flowers have stamens, they are called staminate flowers. When only a pistil occurs, the flower is a pistillate flower. A plant such as corn bears both staminate (tassel) and pistillate (silk) flowers on the same plant and is said to be a monoecious plant. However, in species such as asparagus, and papaya, plants may either be pistilliate (female plant) or staminate (male plant) and are said to be dioecious plants. Flowers may either be solitary (occur singly or alone) or may be grouped together to form an inflorescence. An inflorescence has a primary stalk (peduncle) and numerous secondary smaller stalks (pedicels). The most common inflorescence types in crop plants are the cyme and raceme. A branched raceme is called a panicle (e.g. oats) while a raceme with sessile (short pedicels) is called a spike (e.g. wheat). From the foregoing, it is clear that a plant breeder should know the specific characteristics of the flower in order to select the appropriate techniques for crossing.
5.4.6 Gametogenesis
Sexual reproduction entails the transfer of gametes to specific female structures where they unite and are then transformed into an embryo, a miniature plant. Gametes are formed by the process of gametogenesis. They are produced from specialized diploid cells called microspore mother cells in anthers and megaspore mother cells in the ovary (Figure 5.4). Microspores derived from the mother cells are haploid cells each dividing by mitosis to produce an immature male gametophyte (pollen grain). Most pollen is shed in the two‐cell stage, even though sometimes, as in grasses, one of the cells later divides again to produce two sperm cells. In the ovule, four megaspores are similarly produced by meiosis. The nucleus of the functional megaspore divides three times by mitosis to produce eight nuclei, one of which eventually becomes the egg. The female gametophyte is the seven‐celled, eight‐nucleate structure. This structure is also called the embryo sac. Two free nuclei remain in the sac. These are called polar nuclei because they originate from opposite ends of the embryo sac.
Figure 5.4 Gametogenesis in plants results in the production of pollen and egg cells. Pollen is transported by agents to the stigma of the female flower, from which it travels to the egg cell to unite with it.
5.4.7 Pollination and fertilization
Pollination is the transfer of pollen grains from the anther to the stigma of a flower. This transfer is achieved through a vector or pollination agent. The common pollination vectors are wind, insect, mammals, and birds. Flowers have certain features that suit the various pollination mechanisms (Table 5.1). Insect‐pollinated flowers tend to be showy and exude strong fragrances. Birds are attracted to red and yellow flowers. When compatible pollen falls on a receptive stigma, a pollen tube grows down the style to the micropylar end of the embryo sac, carrying two sperms or male gametes. The tube penetrates the sac through the micropyle. One of the sperms unites with the egg cell, a process called fertilization. The other sperm cell unites with the two polar nuclei (called triple fusion). The simultaneous occurrence of two fusion events in the embryo sac is called double fertilization.
Table 5.1 Pollination mechanisms in plants.
| Pollination vector | Flower characteristics |
| Wind | Tiny flowers (e.g. grasses); dioecious species |
| Insects | |
| Bees | Bright and showy (blue, yellow); sweet scent; unique patterns; corolla provides landing pad for bees |
| Moths | White or pale color for visibility at night; strong penetrating odor emitted after sunset |
| Beetles | White or dull color; large flowers; solitary or inflorescence |
| Flies | Dull or brownish color |
| Butterflies | Bright colors (often orange, red); nectar located at base of long slender corolla tube |
| Bats |
Large flower with strong fruity
|
