Growth Control

The patterns of secondary organ formation are controlled both by genetic factors and by environmental conditions. Horticulturists use plants as a source of dwarfing stocks that have a genetic predisposition to form branches early. In many cases, the dwarfing results from a failure of the stem to elongate in the internodes (the regions between the nodes, where leaves and lateral branches originate). Dwarfing appears to be particularly influenced by plant hormones called gibberellins, which stimulate internodal elongation in dicots. The effect of gibberellins is also influenced by the concentration of the other hormones within the plant.

Likewise, the architecture of columnar plants is under genetic and hormonal control. The Lombardy poplar, for example, has greatly reduced branching compared to the European poplar. This elongation

Control Plant Branching

Many plants as they grow produce secondary organs: branch stems and branch roots. These secondary organs are not derived from the original axis of the plant. The patterns of secondary organ formation determine the architecture of the plant: the shape of the crown and the root system. This architecture plays an important role in the ability of the plant to compete for sunlight, water, and soil nutrients.

Many plants as they grow produce secondary organs: branch stems and branch roots. These secondary organs are not derived from the original axis of the plant. The patterns of secondary organ formation determine the architecture of the plant: the shape of the crown and the root system. This architecture plays an important role in the ability of the plant to compete for sunlight, water, and soil nutrients.

of the principal axis is similar to that found in forest trees growing in the shade of the surrounding forest. The shaded environment stimulates the growth of the main axis of many tree species while inhibiting growth of the secondary stems. As a result, the stem reaches above the surrounding trees and is better able to compete for light.

The inhibition of secondary stem formation seems to be influenced primarily by hormones called auxins. High auxin concentrations inhibit the development of secondary stems, while low auxin levels stimulate the formation of branches. In some species, high cytokinin levels also stimulate secondary stem growth. Because cytokinins are produced in large quantities in the root tips, and auxins are produced in large quantities in the stem tip, the relationship between these two chemicals reflects the balance between the root system and the stem system.

Less is known about the mechanisms of control of branching in the root system. Some species, especially monocots, have many secondary roots of approximately equal size. Others have dominant primary roots, called taproots, with little development of secondary roots. Carrots carry this pattern to an extreme.

The pattern of secondary tissue formation is determined by an interplay between genetic factors and environmental conditions. Many plants complete their life cycles in a single year. This quick pas sage from seed to seed is under genetic control. Annuals rarely develop woody tissue; perennials survive many seasons and show increases in stem girth throughout their lives.

The annual rings seen in the cross-section of a tree are a result of seasonal variations in the production of secondary xylem. Variations in the thickness of annual rings are the result of genetic controls and environmental factors such as mean temperature, damage from insect pests or other pathogens, and water and nutrient availability. Even wind can have significant effects. Strong prevailing winds cause an effect called wind pruning, which results in reduced branching and shorter distances between the annual rings on the windward side.

Secondary tissues are extraordinarily complex. The patterns of cell division, apparently under genetic control, are influenced by a whole concert of hormones. Hormonal gradients and seasonal gradients of sugars and amino acids may play a role in these patterns of secondary tissue formation. The activity of mature phloem tissues and the concentrations of auxins, gibberellins, cytokinins, and perhaps of the gaseous hormone ethylene may all be important in regulating the activity of the cambia.

Craig R. Landgren, updated by Bryan Ness

See also: Angiosperm life cycle; Germination and seedling development; Hormones; Plant life spans; Plant tissues; Roots; Seeds; Shoots; Stems.

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