Branching in Plants (Essay Sample)

Branching in Plants
There are two distinct primary forms of branching, dichotomy and monopodial branching. In dichotomy the growth center divides into two new growth centers, which usually produce second-order branches, almost identical in length and thickness, which can in turn divide into third-order branches, and so on. This form of branching is characteristic of many algae, some fungi, club mosses, liverworts, and other plants. In monopodial branching the growth of the main axis does not stop, and second-order branches, usually less developed, form below the top of the main axis. These branches can also divide into third-order branches and so on. This type of branching is peculiar to spruce, pines, and other coniferous plants, to many herbaceous plants, to leafy mosses, and other plants.
False dichotomy arises from monopodial branching: the growth of the main axis stops and two nearly identical second-order branches, opposite each other, develop below its top and grow beyond the main axis. This may be observed in lilacs (under the inflorescences), the horse chestnut, and mistletoe. Sympodial branching can arise either from dichotomy or monopodial branching. In the first instance one of the branches develops more strongly, growing in the direction and taking on the external appearance of the main axis, and the other branch, which is less developed, becomes more like a branch of the following order. This kind of branching is found, for instance, in selaginella. In the second instance (more widespread) the growth of the main axis stops, and its place is taken by the side branch nearest the top. This kind of substitution can be repeated many times. Sympodial branching is widespread in flowering plants and is inherent in fruit trees and shrubs—lindens, hazels, willows, birch, aspen, rhizomes of grasses, and others. Branching determines the exterior appearance, or habitus, of the plant and is used in taxonomy.
Diagrams of branching: (1) dichotomy, (2) monopodial, (3) sympodial, (4) false dichotomy. The roman numerals designate branches of various orders.
In addition to the stems, the roots, inflorescences, veins (conducting bundles) in leaves and stems, thalli in lower plants, and so forth can also branch. Occasionally shoots appear during branching that are different from the parental shoot (for example, during the tillering of grasses and the formation of runners and stolons)
Sympodial Branching in plants
Laelia superbiens, a sympodial orchid.
In biology Sympodial (literally "with conjoined feet") is the outward morphology or mode of growth of organisms. Plants with sympodial growth have a specialized lateral growth pattern in which the apical meristem is terminated. The apical meristem can either be consumed to make an inflorescence or other determinate structure, or it can be aborted. Growth is continued by a lateral meristem, which repeats the process. The result is that the stem, which may appear to be continuous, is in fact derived from multiple meristems, rather than a monopodial plant whose stems derive from one meristem only.[1]
An example is the orchid Laelia (see illustration). The apical meristem of the rhizome forms an ascendent swollen stem called a pseudobulb, and the apical meristem is consumed in a terminal inflorescence. Continued growth occurs in the rhizome, where a lateral meristem takes over to form another pseudobulb and repeat the process. This process is evident in the jointed appearance of the rhizome, where each segment is the product of an individual meristem, but the sympodial nature of a stem is not always clearly visible.
In angiosperms
The stem is an aerial axis of the plant that bears leaves and flowers and conducts water and minerals from the roots and food from the site of synthesis to areas where it is to be used. The main stem of a plant is continuous with the root system through a transition region called the hypocotyl. In the developing embryo, the hypocotyl is the embryonic axis that bears the seedling leaves (cotyledons).
In a maturing stem, the area where a leaf attaches to the stem is called a node, and the region between successive nodes is called an internode. Stems bear leafy shoots (branches) at the nodes, which arise from HYPERLINK "/EBchecked/topic/83060/bud" buds (dormant shoots). Lateral branches develop either from axillary, or lateral, buds found in the angle between the leaf and the stem or from terminal buds at the end of the shoot. In temperate-climate plants these buds have extended periods of dormancy, whereas in tropical plants the period of dormancy is either very short or nonexistent.
The precise positional relationship of stem, leaf, and axillary bud is important to understanding the diversity of the shoot system in angiosperms. Understanding this relationship makes it possible to identify organs such as leaves that are so highly modified they no longer look like leaves, or stems that are so modified that they resemble leaves.
Branching in angiosperms may be dichotomous or axillary. In dichotomous branching, the branches form as a result of an equal division of a terminal bud (i.e., a bud formed at the apex of a stem) into two equal branches that are not derived from axillary buds, although axillary buds are present elsewhere on the plant body. The few examples of dichotomous branching among angiosperms are found only in some cacti, palms, and bird-of-paradise plants.
The two modes of axillary branching in angiosperms are monopodial and sympodial. Monopodial branching occurs when the terminal bud continues to grow as a central leader shoot and the lateral branches remain subordinate—e.g., HYPERLINK "/EBchecked/topic/58286/beech" beech trees (Fagus). Sympodial branching occurs when the terminal bud ceases to grow (usually because a terminal flower has formed) and an axillary bud or buds become new leader shoots, called renewal shoots—e.g., the Joshua tree (Yucca brevifolia). Plants with monopodial growth are usually pyramidal in overall shape, while those with sympodial growth often resemble a candelabra.
By combining monopodial and sympodial branching in one plant, many different tree architectures have evolved. A simple example is found in HYPERLINK "/EBchecked/topic/168017/dogwood" dogwoods (Cornus), where the main axis is monopodial and the lateral branches are sympodial.
Very different plant forms result from simply changing the lengths of the internodes. Extreme shortening of the internodes results in rosette plants, such as HYPERLINK "/EBchecked/topic/337610/lettuce" lettuce, Lactuca sativa (Asteraceae), in which the leaves develop but the internodes between them do not elongate. Extreme lengthening of the internodes often results in twining vines, as in the yam, Dioscorea esculenta (Dioscoreaceae).
HYPERLINK "/EBchecked/topic/565188/stem" Stems
The stem is an aerial axis of the plant that bears leaves and flowers and conducts water and minerals from the roots and food from the site of synthesis to areas where it is to be used. The main stem of a plant is continuous with the root system through a transition region called the hypocotyl. In the developing embryo, the hypocotyl is the embryonic axis that bears the seedling leaves (cotyledons).
In a maturing stem, the area where a leaf attaches to the stem is called a node, and the region between successive nodes is called an internode. Stems bear leafy shoots (branches) at the nodes, which arise from buds (dormant shoots). Lateral branches develop either from axillary, or lateral, buds found in the angle between the leaf and the stem or from terminal buds at the end of the shoot. In temperate-climate plants these buds have extended periods of dormancy, whereas in tropical plants the period of dormancy is either very short or nonexistent.
The precise positional relationship of stem, leaf, and axillary bud is important to understanding the diversity of the shoot system in angiosperms. Understanding this relationship makes it possible to identify organs such as leaves that are so highly modified they no longer look like leaves, or stems that are so modified that they resemble leaves.
Branching in angiosperms may be dichotomous or axillary. In dichotomous branching, the branches form as a result of an equal division of a terminal bud (i.e., a bud formed at the apex of a stem) into two equal branches that are not derived from axillary buds, although axillary buds are present elsewhere on the plant body. The few examples of dichotomous branching among angiosperms are found only in some cacti, palms, and bird-of-paradise plants.
The two modes of axillary branching in angiosperms are monopodial and sympodial. Monopodial branching occurs when the terminal bud continues to grow as a central leader shoot and the lateral branches remain subordinate—e.g., beech trees (Fagus). Sympodial branching occurs when the terminal bud ceases to grow (usually because a terminal flower has formed) and an axillary bud or buds become new leader shoots, called renewal shoots—e.g., the Joshua tree (Yucca brevifolia). Plants with monopodial growth are usually pyramidal in overall shape, while those with sympodial growth often resemble a candelabra.
By combining monopodial and sympodial branching in one plant, many different tree architectures have evolved. A simple example is found in dogwoods (Cornus), where the main axis is monopodial and the lateral branches are sympodial.
Very different plant forms result from simply changing the lengths of the internodes. Extreme shortening of the internodes results in rosette plants, such as lettuce, Lactuca sativa (Asteraceae), in which the leaves develop but the internodes between them do not elongate. Extreme lengthening of the internodes often results in twining vines, as in the yam, Dioscorea esculenta (Dioscoreaceae).
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