The Hidden Half


Pisum sativum is more commonly known as the pea plant. These plants are well known for growing peas in pods which are the plants fruit.

This is an important crop now grown in many parts of the world. It is thought the pea first originated in the area of modern day Greece, Turkey and Syria.

Despite now being a commonly eaten vegetable when peas were first introduced to France in the 1600s they were described as a luxurious delicacy.

Image showing the root structure of a pea plant.

Peas were also vital to the discovery of modern genetics as Gregor Mendel used simple characteristics, such as shape and colour of the pea seeds, to understand inheritance.

3D Root Architecture

This video shows the developing root system of a 14 day old pea plant.  The shoot and root having developed from inside the pea.  The root architecture developing from one primary root which grows downwards more or less straight.  From this primary root a large number of secondary (or lateral) roots have branched to fill the pot as it has grown.

How do roots know in which direction to grow?

Roots need to grow downwards into the soil (a process called positive gravitropism), and a special set of cells at the very tip of every root tells the root which way is up and down. These cells, called columella cells, contain small starch grains, which are quite heavy compared to the rest of the cell content. Due to their weight, these starch grains will always move to the bottom of the cell. When the primary root grows down straight into the soil, the position of the starch grains is not disturbed. However, if the root is not growing straight down but sideways, for example because a stone is blocking its path, the position of the columella cells changes.
Image showing the gravitropism effect on pea plant and development from 8 days through to 29 days.

What used to be one of the side walls of the cell becomes the bottom, and the starch grains will move towards this point. This movement of the starch grains sets a plant hormone based signalling process in motion that changes root growth. The lower side of the root grows slower than the upper side of the root, which causes the root to bend until the root tip is pointing straight down again. Lateral roots are programmed not to grow down straight like the primary root, but at an angle. This helps the plant explore the soil for anchorage, nutrients, and water.

Image showing the ageotropum effect on pea plant and development from 8 days through to 29 days.
Nature does not however always conform to these natural principals and in the ageotropum pea mutant (right) for example, positive gravitropism does not work. Without gravitropism as the main force for an ordered root (system) structure, the roots are growing randomly through the soil often appearing to defy gravity by growing upwards through the soil system and exposing themselves outside of the soil environment.

Gravitropic Pea

Ageotropum Pea 


In this video we can observe the growth and development of a normal gravitropic pea after 8 days of growth and then its development after 29 days of growth showing the roots behaving in a positive gravitropism.

In this video we can observe the growth and development over the same period as the normal pea however even at the very early stage we can observe the roots do not behave as normal and are not growing in the direction of gravity but are bending upwards towards the soil surface.


The Hidden Half

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University of Nottingham
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