The seedling

Within the seed is a food store that provides the means to produce energy for germination. Once the food store has been exhausted, the seedling must rapidly become independent in its food supply and begin to photosynthesize. It must therefore respond to stimuli in its environment to establish the direction of growth. Such a response is termed a tropism, and is very important in the early survival of the seedling (Figure 11.2).

Direction of light Shoot grows towards light source = positive phototropism

Roots grow away from light source = negative phototropism

Down in response to gravity = positive geotropism

Figure 11.2 Geotropism and phototropism shown as mechanisms assisting the survival of the seedling

Geotropism is a directional growth response to gravity. The emergence of the radicle from the testa is followed by growth of the root system, which must take up water and minerals quickly so that the shoot system may develop. A seed germinating near the surface of a growing medium must not put out roots that grow on to the surface and dry out, but the roots must grow downwards to tap water supplies. Conversely, the plumule must grow away from the pull of gravity so the leaves develop in the light.

Etiolation is the type of growth which the shoot produces as it moves through the soil in response to gravity. The developing shoot is delicate and vulnerable to physical damage, and therefore the growing tip is often protected by being bent into a plumular hook. The stem grows quickly, is supported by the structure of the soil and therefore is very thin and spindly, stimulated by friction in the soil which causes release of ethylene. The leaves are undeveloped, as they do not begin to function until they move into the light. Mature plants that are grown in dark conditions also appear etiolated.

A tropism is a directional growth response to an environmental stimulus.

Greater amount of auxin in shaded half of stem

Greater amount of auxin in shaded half of stem

Negativephototropism

Direction of light Shoot grows towards light source = positive phototropism

Roots grow away from light source = negative phototropism

Down in response to gravity = positive geotropism

Was this article helpful?

0 0
Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

How would you like to save a ton of money and increase the value of your home by as much as thirty percent! If your homes landscape is designed properly it will be a source of enjoyment for your entire family, it will enhance your community and add to the resale value of your property. Landscape design involves much more than placing trees, shrubs and other plants on the property. It is an art which deals with conscious arrangement or organization of outdoor space for human satisfaction and enjoyment.

Get My Free Ebook


Post a comment