Photosynthesisers – Plants – Part 1

Plants are organisms that have a visible part aboveground (the shoot system) and a hidden part belowground (the root system). The extreme variety in the shapes of the visible portion of the plants is also present in the roots below the surface of the soil. The two main types of root systems are fibrous and taproot. Fibrous roots are the traditional structures formed by primary and secondary roots branching in all directions in the soil. By contrast, taproots are characterised by a single firm root growing straight down, with minor roots developing either side of it. Other specialised roots do exist; for example, the tuberous roots of sweet potato are modified for the storage of nutrients and water, while the stilt roots of mangroves allow the plant to be stable in wet and muddy soils by cropping up from the trunk and growing downwards. Roots are usually covered by root hairs that are invisible to the naked eye and form a large surface area allowing plants to take up water and mineral nutrients from the soil.

Green plants (Viridiplantae), are a kingdom of organisms including from 300 000 to 315 000 different species. The majority, 260 000 to 290 000 species, produce seeds. The two main groups of seed plants are the flowering plants (Angiosperms) and the naked-seed plants (Gymnosperms). Angiosperms produce fruits containing seeds and include the most common vegetables and fruits used as food by humans. Angiosperms comprise monocotyledons (e.g. grasses, such as maize or wheat) that have fibrous root systems, and dicotyledons (e.g. carrots and apples) that have taproot systems. Gymnosperms include the conifers, which are woody plants with cones and root structures similar to those of dicotyledons.

Plants are found everywhere, from tundra to desert. The aboveground parts of plants are responsible for the photosynthesis that provides energy for the plants and
replenishes oxygen in the atmosphere. By contrast, the root system has three main functions:
1) absorption of nutrients and water;
2) anchorage to soil;
3) storage of nutrients.
Plant roots generally grow anywhere with suitable environmental conditions and readily explore soil macropores. The part of the soil that is directly influenced by roots is called the rhizosphere, and is very rich in soil microorganisms (e.g. in bacteria and fungi).


Incredible numbers of plant roots

The maximum rooting depth, 68 metres, was found in a plant in the Kalahari Desert.

A single winter rye plant (Secale cereale) can grow roots measuring 620 kilometres in only 0.5 cubic metres of soil.

A grove of over 40,000 clonal quaking aspens (Populus tremuloides), located in south-central Utah (USA), has the largest root system in the world. It is estimated to weigh 6,600 tonnes.[/box]

Ref: Global Soil Biodiversity Atlas p. 45