Branching in stems with (diagram)


The below- or above-ground stems or shoots of a plant often exhibit characteristic branching patterns. Branching pattern is determined by the relative activity of apical meristems, both the original shoot apical meristem derived from the seedling epicotyl and apical meristems subsequently derived from lateral buds.

One major feature of branching pattern has to do with the duration of apical meristematic growth of a shoot. If a given shoot has the potential for unlimited growth, such that the apical meristem is continuously active, the growth is termed indeterminate.

If instead a shoot terminates growth after a period of time, with either the abortion of the apical meristem or its conversion into a flower, inflorescence or specialized structure (such as a thorn or tendril), the growth is termed determinate. (Note that these same terms are used for inflorescence development; see later discussion.)

Two other, related terms have to do with flowering. A determinate shoot that completely transforms into a flower or inflorescence is called hapaxanthic. An indeterminate shoot that bears lateral flowers but that continues vegetative growth is termed pleonanthic.

Many different models of stem branching pattern have been described (e.g., see HallØ et al. 1978). These models may be of taxonomic value and are interesting from a bio-mechanical perspective, as they may represent evolutionary adaptations to a given environment or life strategy.

Monopodial Branching

Three general terms focus on the developmental origin of a given branch or axis. If a given stem axis is derived from growth of a single apical meristem, the pattern is termed monopodial. The monopodial axis may grow indefinitely and thus be indeterminate.

Sympodial Branching

In contrast, if a given axis (which may appear to be a single, continuous structure) is made up of numerous units that are derived from separate apical meristems, the branching pattern is sympodial. These sympodial units arise from lateral buds that are proximal to the apical meristem of the original shoot.

Many rhizomes have sympodial growth. Finally, a rare type of branching is dichotomous, in which a single apical meristerm divides equally into branches, e.g., Psilotum.

                                                                     Figure . Stem branching patterns.
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