20 Plant Growth Characteristics
Learning Objectives
- Recognize and describe plant growth characteristics.
Plant form and growth habit are among the most noticeable and important features for identification purposes as well as for landscape plant selection. Plant form, the three-dimensional shape or silhouette outline of a plant, is determined by the habit or branching pattern. For example, plants with an excurrent growth habit have single, undivided trunks and lateral branches that typically produce an overall cone or pyramid-shaped form. This plant form and growth habit is characteristic of many gymnosperms such as Thuja plicata (western red cedar) and Pseudotsuga menziesii (Douglas fir). In contrast, decurrent, or sometimes called deliquescent growth habit exhibits several roughly equal branches arising from the trunk or stem that become the main structural system of the plant. This habit results in the typical rounded or spreading form of deciduous trees such as Acer macrophyllum (big leaf maple) or Acer platanoides (Norway maple) as well as many shrubs. Depending on the branching pattern, additional descriptive terms such as upright or horizontal, arching or weeping, open, twiggy or dense may be used for shrubs as well as for trees.
Stem, Bark, and Bud Morphology
In addition to plant form and habit, the winter identification of deciduous trees and shrubs depends on the morphology of stems, bark, and buds. Stem color, surface texture and the presence of lenticels, small cork-like spots for gas exchange between plant tissues and air are characteristic for some species of Prunus (cherry). A cross section taken through a stem or shoot reveals soft plant tissue, the pith. The color and texture of the pith may be used for distinguishing between similar plant types, such as species of Cornus (dogwood). Pith may be brown or white, variably shaped, and uniformly solid, chambered or hollow as illustrated in Figure 20.1.
Bark, the dead outer protective tissue of woody plants can vary greatly in appearance, thickness, and texture as a tree or shrub matures. In addition to plant identification, plant bark may have highly ornamental value in the landscape. Color change, peeling and exfoliation, and smooth, or furrowed (grooved), ridged or plate-like are some common descriptors used for bark. For example, Figure 20.2 shows the bark of Platanus × acerifolia[New Tab] . In addition to its ornamental value, the bark is a key identification feature for this tree.
Buds, condensed shoots containing a new leaf, leaf cluster, or flower are located in leaf axils and at tips of stems. In general, a flower bud appears somewhat larger and rounder than a vegetative bud. While bud shape, size, color, and surface texture vary by species, bud arrangement will be alternate, opposite or whorled on the stem. Bud scales, the protective covering of buds may be single, few, or many, and imbricated (overlapping) or not as shown in the Figure 20.3. The shape of a leaf scar, where