Functions of water

The plant consists of about 95 per cent water, which is the main constituent of protoplasm or living matter. When the plant cell is full of water, or turgid, the pressure of water enclosed within a membrane or vacuole acts as a means of support for the cell and therefore the whole plant, so that when a plant loses more water than it is taking up, the cells collapse and the plant may wilt. Aquatic plants are supported largely by external water and have very little specialized support tissue. In order to survive, any organism must carry out complex chemical reactions, which are explained, and their horticultural application described, in Chapter 8. Raw materials for these chemical reactions must be transported and brought into contact with each other by a suitable medium; water is an excellent solvent. One of the most important processes in the plant is photosynthesis, and a small amount of water is used up as a raw material in this process.

  • Diffusion is a process whereby molecules of a gas or liquid move from an area of high concentration to an area where there is a relatively lower concentration of the diffusing substance, e.g. sugar in a cup of tea will diffuse through the tea without being stirred -eventually! If the process is working against a concentration gradient, energy is needed.
  • Osmosis can therefore be defined as the movement of water from an area of low salt concentration to an area of relatively higher salt concentration, through a partially permeable membrane. The greater the osmotic pressure then the faster water moves into the root cells, a process which is also affected by increased temperature.
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