Chapter 18. Chapter 18: Growth and Reproduction in Plants

In the Orkneys, aspens reproduce asexually. One tree can give rise to dozens or even hundreds of genetically identical offspring.

By planting a potato adapted to its particular environment, a farmer can preserve the same, well-adapted set of alleles as in the parent plant. The farmer will choose for planting those potatoes that have the characteristics she wants to propagate in the next crop. The seeds would most likely have a different combination of alleles.

Sexual reproduction in plants is important in agriculture because it allows the selective breeding of particular plants for sought-after features such as size, flavor, or speed of ripening.

The ovule develops into a seed.

Plants release billions of tiny pollen grains each spring and summer. The proteins that project from the cell wall of each pollen grain increase the stickiness of the pollen, which helps in fertilization, but they can also act as allergens (proteins detected as foreign substances and producing an allergic reaction) when people breathe in the pollen grains.

Fertilization occurs in the embryo sac, which is located in the ovary of the female flower.

There has been strong coevolution between plants and their animal pollinators. Plants have become increasingly effective at attracting pollinators and deterring other species from visiting their flowers, while pollinators have become more effective at exploiting the resources offered by the plants.

The fruits or seeds are the most calorie-rich parts of a plant.

Double fertilization involves two separate fusions of male nuclei with female nuclei—one producing the embryo and the other producing the endosperm. This process is efficient because when, and only when, an embryo is produced, so, too, is a ready-made food source.

Flowers commonly contain both male and female reproductive parts, so a pollinator can easily assist in sexual reproduction. This hermaphroditism—having both male and female reproductive parts—in flowering plants also allows self-fertilization. One of the pros of self-fertilization is that the plant is able to reproduce in the absence of pollen arriving from another plant. One of the cons is that the offspring will be less genetically diverse than those produced by cross-pollination and are more likely to express an undesirable gene (because of inbreeding).

Once the fruit has formed, the seeds are protected and are in a “waiting” mode, waiting for suitable circumstance in which to germinate. No further metabolic activity is required until germination. The fruit begins to dry in preparation for dispersal.

Fruits do not ripen and become sweet and edible until the seeds are fully developed—only then are the seeds ready for dispersal. At that point, the fruit sweetens and turns a bright color to attract the attention of dispersers.

A seed contains an embryo and a supply of nutrients to fuel its growth on germination, when the conditions are right. No light (i.e., no photosynthesis) is necessary for this early stage of growth.

A plant will grow taller (undergo primary growth) if its lower branches are removed, or pruned (but not overly pruned), to reallocate resources to growth of the unpruned portions. This growth occurs through the activity of apical meristems.

As a plant shoot grows, some meristem cells are left behind in a dormant state. These meristem cells can begin dividing at any time, pushing outward and forming branches.

Wood (a) conducts water from the roots to the leaves, and (b) provides support for the plant.