When plants evolved from marine species and made their way onto land they had to overcome many challenges to survive and thrive on land. One significant difference between water based environments, such as lakes and oceans, and living on the land was the separation of nutrients. Plant roots were a key adaptation for plants on land to allow access nutrients and water stored in soil.
Living in water, plants are able to collect CO2, water and other nutrients straight from the surrounding water. On land however, these essential nutrients are separated between the atmosphere and the soil. Over time, plants evolved two main systems to collect sufficient nutrients from the two different environments.
Plant roots, found almost entirely underground, is their way of collecting water and nutrients essential for growth and survival. The roots of a plant perform a range of services that are essential to the survival of any land plant; they absorb water and nutrients from the soil, help to anchor the plant to the ground and often store large quantities of food.
Fibrous and tap roots
A plant’s root system can be either fibrous or have a distinct tap root. Many dicot plants have a main root known as the tap root which has many lateral roots growing from it. By having a thick tap root that grows deep into the soil, the plant gains extra anchorage to the ground. Some plants species have extra thick tap roots such as carrots and parsnips that store large amounts of nutrients.
Fibrous root systems, common in monocots, have many very thin roots spread out under the surface and form a mat of roots underground. Keeping their roots close to the surface means they may lose some stability provided by the deep tap root, however the large number of roots firmly secure the plant to ground. Plant species with fibrous roots such as grasses are also great at stabilising the soil and preventing erosion. The extensive root system provides excellent exposure to nutrients and water in the soil.
Plant roots are important for securing the plant to the soil but the majority of nutrient uptake occurs near the tip of the root. Here a large number of root hairs grow; very fine roots with large surface area to volume ratio. By having a large surface area and low volume it increases the efficiency of absorption of minerals and water.
A common feature of almost all root systems is what is known as mycorrhizae. A mycorrhizae is a relationship between the roots of a plant and fungi where both species usually benefit. The plant supplies the fungi with a constant source of sugars and the superior absorption abilities of the fungi help to provide the plant with an increased supply of water and nutrients.
Carbohydrates produced through photosynthesis in the plant’s leaves and stem are transported down to the root tissue and the fungi. The fungi are far smaller than any root hair and therefore have a much smaller surface area to volume ratio and are much more efficient at absorbing nutrients such as nitrate, ammonium and phosphate. Mychorrizal partnerships are most beneficial in nutrient poor soils.
It is common knowledge that roots grow below ground, but in certain cases, plants will grow roots above ground from stems and even leaves. Plant roots such as these are labelled adventitious, a term used to describe a structure that grows in a strange place.
The banyan tree is a great example of a species with adventitious roots, which have a large proportion of their roots above ground. As it begins its life in the branches of a host tree, the young banyan tree germinates and grows its roots down to the soil, often wrapping itself around the host tree on the way down. Many mangrove species also grow adventitious roots from stems to provide support in an environment of constantly changing tide height and mangrove fruit germinate on the tree and begin to grow an adventitious root while they are still connected to the parent plant.
Although roots can come in many forms and develop in a number of different ways, in nearly all circumstances their purpose remains constant and were developed as a mechanism to collect nutrients and water in a land based environment.
Last edited: 18 January 2016
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