LONG SHOOT-SHORT SHOOT

An easily recognized shoot design is termed long shoot-short shoot organization, wherein the canopy consists or two distinct classes of shoots: (1) major axes that have long internodes, and (2) lateral axes that have no internodes and therefore appear as very short projections. The classic example of long shoot-short shoot organization is the deciduous gymnosperm Ginkgo biloba, the maidenhair tree (Family Ginkgoaceae).

Some plants have long shoots and spur shoots, on which flowering often occurs. A spur shoot tends to have some internodal elongation, hence its distinction from short shoots, which lack internodal elongation. Some authors describe short shoots as having "fascicled leaves."

The molecular triggers for long shoot-short shoot organization need to be carefully studied to determine what genes are involved. Studies from the previous generation of plant physiologists observed that different treatments of growth substances (plant hormones) can be used to cause a plant to produce long shoots versus short shoots, and internodal growth is apparently suppressed when a lower than normal level of auxin is present.

Long shoot-short shoot organization has not been studied intensively from an ecological and evolutionary perspective. In desert and semiarid, full sun habitats is where many examples of long shoot-short shoot organization can be observed. There are several possible benefits of this design.

  1. Especially in deserts, a dormant shrub or tree can produce a crop of photosynthesizing leaves within several days following an effective rainfall event. Growth is more rapid because, basically, no stem tissues need to be formed.
  2. Woody plants with long shoot-short shoot organization, because they have fewer branches, tend to permit sunlight to penetrate deeply through the plant, rather than only striking the uppermost canopy and canopy margins.
  3. Inflorescences are formed on the short shoots, a short distance from the leaves supplying the nutrients.
  4. Leaves tend to have little development of petioles and are clustered so close to the stem that there may be some benefits in avoiding wind shear of the leaves.

Examples of Long Shoot-Short Shoot Organization

Among gymnosperms, there are four types of long shoot-short shoot organization found in living (extant) species.

  1. deciduous needles in the genera Larix (larches) and Pseudolarix (golden larch) (Family Pinaceae)
  2. evergreen needles in the genus Cedrus (Family Pinaceae)
  3. the maidenhair tree, Ginkgo biloba (Family Ginkgoaceae), which has distinctive fan-shaped (flabelliform) leaves, sometimes deeply bilobed
  4. pines (Pinus, Family Pinaceae), which has determinate short shoots, the "fascicle" of needles, one to five photosynthetic leaves and several scale-like leaves wrapping the bases of the leaves; the trees are evergreen, and the short shoots abscise after two or more growing seasons

Some characteristic spiny plants of deserts and dry tropical habitats display long shoot-short shoot organization.

  1. The areole of a cactus is a determinate short shoot, which develops immediately and directly (sylleptic development) in the axil of the leaf. Therefore, an areole is the equivalent of an axillary bud that continues to grow. The spines of a cactus are formed from the shoot apical meristem on the areole in helically alternate phyllotaxis, i.e., they are equivalent to alternately arranged leaf primordia that mature instead as hard spines, leaf spines. The areole remains viable often for many years, typically until the apical meristem forms a flower, when the living cells are used up and the areole then dies after the fruit finishes its growth.
  2. The ocotillos (Fouquieria, Family Fouquieriaceae), a North American family occurring in deserts and dry tropical forests and scrubland of Mexico and the southernmost United States, has spreading wand-like long shoots and small leaves clustered on short shoots. The fouquieriaceous spine forms from hard cells on the lower half of the leaf petiole and midvein. Leaves are produced within a few days after a soaking rain or irrigation, and abscise when the plant experiences a mild water stress.
  3. The didiereas (Family Didiereaceae), found only on Madagascar, are superficially very similar to the North American ocotillos (just described), but these families are not closely related, so the wand-like stems and long shoot-short shoot organization are here an example of convergent evolution. In Didiereaceae, drought-deciduous foliage leaves are produced just above the stem spine.

Some desert and dryland species of Family Euphorbiaceae have indeterminate short shoots, which form leaves year after year.

  1. Euphorbia misera, a common low shrub of the western Sonoran Desert
  2. Species of Jatropha, such as J. spathulata, J. cinerea, and J. cardiophylla are species of North American deserts that have flexible long stems and knobby short shoots

Some trees of deserts and other dryland habitats are excellent examples of long shoot-short shoot organization.

  1. elephant tree (Pachycormus discolor, Family Anacardiaceae) of the desert of Baja California, Mexico
  2. members of the frankincense family (Burseraceae), including certain desert species in the Sonoran Desert of North America (e.g., Bursera microphylla) and deserts of Africa and Arabia (e.g., Commiphora)
  3. microphyllous legumes may be trees or shrubs with short shoots
  4. Oxalis gigantea (Family Oxalidaceae), a peculiar shrub of the fog desert zone in Chile
  5. an assortment of small-leaved shrubs of deserts; in North America, e.g., Porlieria angustifolia, several species of Lycium, Condalia globosa, and numerous others

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