Growth & Sensitivity

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Germination

Growth is the process by which a plant increases the number and size of its roots, leaves and stems.

Growing requires energy. Plants store an energy source in their seeds, so they can grow. When a seed is in the right conditions for growth - with water, warmth and oxygen - it can germinate and grow into a new plant.

First, the seed absorbs water. A young root called a radicle starts to grow downwards through the soil. Next, the seed coat splits and a tiny shoot straightens. The main root becomes bigger, and side roots appear, as well as leaves. The young plant grows more leaves. As new growth increases the leaf size, there is a greater area over which light can strike the leaf. This leads to an increase in food production, allowing still more leaf growth.

Growth in Response to the Environment

Growth that causes parts of the plant to bend towards or away from a stimulus such as light, water, and gravity are called tropisms. Tropisms are the growth responses that plants exhibit in response to their environment.

The stem tips grow towards the direction of sunlight (phototropism).
The root tips grow downwards in response to the pull of gravity (geotropism / gravitropism).
Root tips grow towards high water content areas in the soil (hydrotropism).
Stems respond to touch by twirling around objects (thigmotropism).

Growth at the Meristem

Parts of plants above ground grow up and out. Although all plants of a species look similar, they do not have a 'set plan' that says a branch has to grow in a certain place, or in a certain direction. Compare this with animals, which have body parts in set places.

Growth upwards occurs in plants by cell division and cell elongation. Cells in the apical meristem at the top of a plant stem divide in two by mitosis. One cell stays in the apical meristem while the other cell elongates. Having several cells growing longer increases the length of the stem, making the plant taller.

Older plants grow outwards by increasing the thickness of the stem. Mitosis takes place in the vascular cambium. Cells produced on the inside of the vascular cambium become xylem cells, while some of those on the outside become phloem cells.

The increase in the number and size of xylem and phloem cells makes the stem thicker.

More on the Cell Theory

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