Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 220

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 pedicels; dull or pale colors; strong fruity or musty scents, flowers produce copious, thick nectar
Birds	Bright colors (red, yellow); odorless; thick copious nectar

On the basis of pollination mechanisms, plants may be grouped into two mating systems: self‐pollinated or cross‐pollinated. Self‐pollinated species accept pollen primarily from the anthers of the same flower (autogamy). The flowers, of necessity, must be bisexual. Cross‐pollinated species accept pollen from different sources. In actuality, species express varying degrees of cross‐pollination, ranging from lack of cross‐pollination to complete cross‐pollination.