Vegetative Growth and Development

Shoots

Diagram of a grass shoot Cross section of a grass leaf blade

The grass shoot is composed (see diagram right), in part, of leaves that differentiate into flattened blades and folded or rolled sheaths. Stems may not be visible, as they are often hidden within one or more enclosing leaf sheaths; thus, an aerial shoot may appear to be made up entirely of leaves.

A cross section of a grass leaf blade (see right) reveals loosely arranged mesophyll cells and a series of parallel veins, all enclosed within a single layer of epidermal cells. Within both the upper (adaxial) and lower (abaxial) epidermis, stomatal openings occur connecting the internal environment of the leaf with the external environment of the atmosphere. It is through these stomatal openings that gas exchange occurs between the leaf and the atmosphere. Water transported from the roots comes into the leaves via the veins, and diffuses across the leaf as films of water surrounding the mesophyll cells. Depending upon environmental conditions, including temperature, relative humidity, and wind speed, these films of water can evaporate, exiting the leaf as water vapor through the stomatal openings. This process is called transpiration. The change of water—from a liquid to a gas—absorbs heat energy; thus, enabling the plants to maintain themselves within physiological temperatures, providing sufficient water is available from the soil.

Diagram showing the crown in the stem of a grass plant

The stem present at the base of an aerial shoot may be invisible if it is entirely enclosed within one or more leaf sheaths. Careful dissection of an aerial shoot, however, can reveal the presence of a highly compressed stem, called a crown.

diagram of a stomate

The grass crown (see diagram left) is an unelongated stem with its growing point situated at the top. The growing point contains leaf primordia that develop into fully expanded leaves. Axillary buds are located along the sides of the crown, and give rise to tillers emerging from intravaginal branching, or lateral shoots emerging from extravaginal branching. Where these lateral shoots grow above the soil surface, they are called stolons, where they grow below ground, they are called rhizomes. The internodes are so short that the crown appears to be composed of a series of nodes stacked one on top of the other. In young crowns, the primary root may still be present; once it reaches about six months in age, however, it disappears, leaving only the adventitious roots that arise from the stem nodes.

diagram of the growing point at the top of the crown

The growing point at the top of the crown continually forms leaf primordia, which eventually develop into fully expanded leaves. Leaf primordia arise due to cell division just below the apical meristem. Rapid division of cells at the midpoint of each leaf primordium results in the formation of the leaf tip. Subsequent meristematic activity is restricted to the basal portion of the leaf primordium, establishing the intercalary meristem (depicted right).

photo of the growing point at the top of the crown illustration showing the youngest leaves on a turfgrass shoot

The number of leaf primordia visible at any time varies from a few to as many as twenty or more, depending on species, plant age, and environmental conditions. The entire length of the growing point is usually less than one millimeter.

The oldest leaves of a turfgrass shoot are located at the base of the shoot, while the youngest leaves are at the uppermost position. The youngest leaves grow most rapidly, while the older leaves eventually cease growing. Thus, the photosynthate requirements of the leaves vary with their age.

While intravaginal growth builds shoot density close to the parent shoots, aggressive extravaginal growth can extend the plant population well beyond the parent shoots and contribute to turfgrass coverage over a large area. Considering the potential impact of these two growth processes, one can visualize how an entire lawn could theoretically develop from a single seedling.

A rhizome is a lateral shoot that grows beneath the ground via stem internode elongation. When a determinate rhizome reaches the ground surface, light interception results in the cessation of internode elongation along with the formation of a new aerial shoot. A daughter plant then develops. Indeterminate rhizomes are long and tend to branch at the nodes, and aerial shoots arise from auxiliary buds along these submerged shoots. Unlike roots, which grow by adding cells at the tip, a rhizome grows by intercalary meristematic activity in the vicinity of the stem nodes, and the subsequent expansion and differentiation of the newly created cells; the resulting elongation of the stem internodes is partly responsible for pushing the rhizome tip through the soil. The conical tip of the rhizome is actually formed by bladeless leaves called cataphylls that are produced at the growing point in somewhat the same fashion as leaves in an aerial shoot. As the leaves form, they also aid in pushing the rhizome through the soil.

In contrast to the rhizome, a tiller grows upward and within the enclosing sheaths of the parent plant. This results in an increase in the number of plants immediately adjacent to the parent plant.

diagram showing tillering and rhizome growth patterns diagram showing a stolon and a rhizome on a turfgrass plant diagram showing the growth pattern of a rhizome and its parts
photo of grass plants with parts labeled

In summary, new leaves emerge from growing points located atop crowns within enclosing leaf sheaths of aerial shoots, or from nodes along lateral shoots—stolons and rhizomes—growing above or below the surface. New shoots arise from axillary buds at the base of each leaf, taking the form of tillers, stolons, or rhizomes. Turfgrasses that spread only through tillering (i.e., intravaginal growth) have a bunch-type growth habit, sometimes forming distinct clumps within a turfgrass community. Rhizomatous and stoloniferous turfgrasses, on the other hand, can spread far from the mother plant, filling in voids in a turf resulting from biotic (e.g., diseases, insects, and animal pests) or abiotic (e.g., desiccation, traffic-induced injury) stresses.

Roots

While both seminal and adventitious roots may be observed growing from turfgrass plants, the root system associated with a mature turfgrass community is expected to be entirely adventitious, as seminal roots—arising from the embryo—usually do not persist for more than six months following germination. Adventitious roots (below left) grow from crown nodes, as well as nodes along rhizomes and stolons.

The root is composed of an organized arrangement of cells produced by division of meristimatic cells at the root tip, which is located just behind the root cap. With the emergence of root hairs, the root's capacity to absorb water and nutrients is dramatically increased. In some species, only specialized epidermal cells, called tricoblasts, can form root hairs. The root tip is protected by a root cap formed by meristematic cells at the root tip. The root cap (below middle), which is unconnected to the root's vascular system, is composed of loosely arranged cells that are completely renewed every six to nine days. In addition to protecting the root tip, the root cap regulates geotropic curvature of the root as it grows through the soil. Also, the root cap produces a “slime sheath”, which lubricates the soil around the developing root, favors the growth of mycorrhizae, increases the availability of nutrients from the soil, promotes soil aggregation, and reduces the root’s susceptibility to desiccation during dry periods.

diagram of a turfgrass plant showing the adventitious roots diagram of a root tip diagram showing the longitudinal section of a turfgrass root

This longitudinal section of a turfgrass root shows the “stele” or central core of the root (above right), the surrounding cortex and epidermis, and other anatomical features. The stele contains the vascular tissues through which materials flow between the roots and their associated shoots. Specific vascular tissues include the phloem through which photoassimilates and some phytohormones are transported to the roots from the shoots, and the xylem through which water, dissolved nutrients, and some phytohormones are transported from the roots to the shoots.

diagram of the cross section of a root

The root cross section provides another perspective that is helpful for understanding the structure and functions of various anatomical features within the root. The epidermis is the surface layer of thin-walled cells surrounding the root. These cells give rise to root hairs which are extensions of the epidermal cell walls. The permeability of the epidermis depends on the degree of suberization (i.e., the accumulation of a waxy substance, called suberin, in the cell walls) that has occurred. Older epidermal cells become increasingly suberized and, as a consequence, increasingly resistant to the movement of water and dissolved nutrients into the root. As water is pulled into the epidermal cells of the root, it moves across the cortex through three distinct pathways: the intercellular spaces between cortex cells, the pore spaces within the cortex cell walls, and through the cortex cells. Movement through the intercellular spaces and the pores within the cortex cell walls is relatively rapid and, because it is through the non-living portions of the root, is called apoplastic movement. Movement thought the cortex cells is slower, requiring transport through the living portion of the cells; this is this called symplastic movement. When water is lost by transpiration from the leaves, a hydraulic tension is created in the xylem that extends throughout the length of the plant, reaching the extent of the root system.