5.3 - Fractal Root Systems
20. Building a Rhizotron and Germinating Seeds
21. Growing Roots
roots are classic examples of plant life. Every plant has a submerged
branching system designed to support its life functions. But why
do roots branch? And why do they branch at so many different levels,
that is, why are there branches of branches? Is this the most efficient
method for a plant to obtain nutrition?
Root growth occurs by elongation of the root tip. Cells in the growing root divide in a region of tissue known as the meristem at the interior of the root. Growth begins as soon as the seedling germinates. The root that appears from the seedling or embryonic plant is known as the primary root. However, if you have ever noticed a plant or seedling after a few weeks, there is more than one vertical root; these other roots are called secondary roots, and branch from the original primary root. But roots do not all grow straight in a vertical direction; many parts of the root extend sidewise from the primary or secondary roots. These roots are known as lateral roots and originate from the same tissues as the primary root.
Many factors affect the growth of plant roots. Of particular interest is the resistance due to the physical barrier of the soil. Properties like the compactness of the soil can alter root growth. For example, in compact soils, the spaces between soil particles, are reduced in both number and size. The space size is usually referred to as pore size. To force their way through compact soil, roots thicken and their rate of growth-elongation-decreases. Their branching patterns become modified and often there appears to be an increase in lateral root growth. (The lateral roots are often smaller in diameter than the primary root, and are better able to grow through tighter pores.) There is much controversy about the variety of mechanisms plants develop in response to soil resistance.
Many other environmental variables affect root growth. In poorly aerated solutions, roots tend to grow straighter, shorter and to generate more lateral roots. In corn plants, light also alters root growth. Typically, blue light inhibits cell elongation and multiplication, while red light inhibits only cell elongation.
In carrying out the experiments described below, you play detective and try to understand some of the factors that give rise to root structure. You can use what you learned earlier about diffusion and branching in non-living systems. Similar branching structures are observed in snowflakes and during crystallization of minerals. In each case it is a diffusion process that controls the rate at which growth occurs. The branching pattern apparently results from this diffusion. For growing crystals, minerals diffuse toward the growing surface. For snowflake growth, heat is released as the ice forms and this heat must diffuse away from the growing surface. For root growth, nutrients must diffuse toward the growing root. This diffusion takes place in a solution for hydroponic growth (growth in fertilized liquids), and in the soil.
It is difficult to go out into the field to do experiments on roots, because when plant roots are removed from the soil they are disrupted. One solution to this problem is adopted in the experiment described below: to use a rhizotron, a clear-walled chamber through which to observe roots as they grow.
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