Poaceae

Growth Form and Habit

Grasses are predominantly herbaceous plants, though some bamboos develop woody culms that can reach heights of 30 meters or more. Most species are perennial, with extensive underground rhizome systems or dense, fibrous roots that allow them to persist through adverse conditions and quickly regenerate after disturbances such as fire or grazing. Annual grasses complete their life cycle in a single growing season and are common in environments with pronounced seasonal variation or frequent disturbance.

The growth habit of grasses is characterized by their ability to grow from the base rather than the tip of their structures (intercalary meristem), allowing them to continue growing even after the upper portions have been removed by grazing or mowing. This adaptation, combined with the protection of growing points within leaf sheaths or close to the ground, has contributed significantly to the family's success in grazed ecosystems.

Grasses exhibit several distinctive growth forms:

• Caespitose (tufted): Forming dense clumps or tussocks
• Stoloniferous: Producing horizontal stems (stolons) that grow along the ground surface
• Rhizomatous: Developing underground horizontal stems (rhizomes)
• Decumbent: With stems that grow along the ground but turn upward at the tips
• Climbing: Some bamboos and tropical grasses climb using specialized hooks or tendrils

Stems (Culms)

The stems of grasses, called culms, are typically cylindrical, hollow between the nodes (solid in some species, particularly corn and sugarcane), and jointed. The nodes are solid and often swollen, representing points where leaves attach and where branching can occur. In many species, the lower portions of culms are enclosed by overlapping leaf sheaths, providing protection and support.

Bamboos represent a specialized growth form within Poaceae, developing woody culms that persist for multiple years. These culms emerge from the ground at their full diameter and elongate rapidly, sometimes growing more than a meter per day. The culms are typically hollow with prominent nodes and achieve their full height in a single growing season, after which they develop branches and leaves.

Leaves

Grass leaves have a highly distinctive structure that sets them apart from most other plant families. Each leaf consists of three main parts:

• Sheath: The lower portion that wraps around the culm, often overlapping itself. The sheath provides protection for developing tissues and emerging inflorescences.
• Blade: The expanded, typically flat portion extending away from the culm. Grass blades are usually linear with parallel venation and grow from the base rather than the tip.
• Ligule: A membranous or hairy appendage at the junction of the sheath and blade on the inner surface. The ligule prevents water and debris from entering the space between the culm and sheath and is an important diagnostic feature for identifying grass species.

Additional leaf features may include:

• Auricles: Claw-like extensions at the base of the blade
• Collar: The outer junction of the sheath and blade
• Keel: A prominent midrib on the underside of the blade

Grass leaves are arranged alternately on the culm in two vertical rows (distichous phyllotaxy). The leaf blades may be flat, folded, rolled, or needle-like, often with adaptations to conserve water or resist damage from wind. The epidermis contains specialized cells, including silica cells that accumulate silicon dioxide, contributing to the toughness of grass leaves and their resistance to herbivory.

Inflorescence

The grass inflorescence is a complex structure with a distinctive organization. The basic unit is the spikelet, which consists of one to many florets (small flowers) arranged on a central axis (rachilla) and enclosed by a pair of bracts called glumes. The spikelets are then arranged in various patterns to form the overall inflorescence, which may take several forms:

• Spike: Spikelets attached directly to the main axis without stalks (e.g., wheat, barley)
• Raceme: Spikelets attached to the main axis by short stalks
• Panicle: A branched inflorescence with spikelets at the ends of branches (e.g., oats, many lawn grasses)
• Digitate: Several spike-like branches radiating from a common point (e.g., crabgrass)

Some grasses have specialized inflorescences, such as the "cob" of corn (a modified spike with female flowers) or the complex branching systems of bamboos, which may flower only once after many years of vegetative growth.

Flowers

Grass flowers (florets) are highly reduced and specialized, lacking the showy petals and sepals found in many other flowering plants. Each floret typically consists of:

• Lemma: A lower bract that often bears an awn (a bristle-like extension)
• Palea: An upper bract that together with the lemma encloses the reproductive organs
• Lodicules: Small, scale-like structures (usually two) that represent the reduced perianth and swell to force open the lemma and palea during flowering
• Stamens: Usually three (occasionally one, two, or six), with versatile anthers that dangle from the floret to facilitate wind pollination
• Pistil: A single carpel with an ovary containing one ovule, and typically two feathery stigmas that are well-adapted to catch wind-borne pollen

Most grasses are wind-pollinated (anemophilous), producing large quantities of lightweight pollen that can be carried long distances. The feathery stigmas provide a large surface area for capturing pollen. Some tropical forest grasses are insect-pollinated, with more colorful and scented flowers.

Fruits and Seeds

The fruit of grasses is a specialized type called a caryopsis (grain), in which the seed coat is fused to the fruit wall (pericarp). This single-seeded, dry, indehiscent fruit is the characteristic form in most grasses, though some produce berries (e.g., some bamboos) or other modified fruit types.

The grass seed contains a large endosperm that serves as a food reserve, a single cotyledon modified into a specialized structure called the scutellum, and an embryo with a shoot apex (plumule) and root apex (radicle). The embryo is positioned laterally at the base of the grain rather than centrally as in many other plant seeds.

Grass fruits exhibit various adaptations for dispersal:

• Awns that twist with changes in humidity, driving the seed into the soil
• Barbed bristles that attach to animal fur or feathers
• Lightweight structures for wind dispersal
• Fleshy fruits in some bamboos that attract animal dispersers

The grains of cereal crops have been selectively bred for increased size, nutritional content, and ease of harvest, making them among the most important food sources for humans worldwide.

Physiological Adaptations

Many grasses possess specialized physiological adaptations that contribute to their ecological success:

• C4 photosynthesis: Approximately 46% of grass species use the C4 photosynthetic pathway, which is more efficient than the more common C3 pathway under conditions of high temperature, high light intensity, and water limitation. C4 grasses dominate in tropical and subtropical grasslands.
• CAM photosynthesis: A few grass species in extremely arid environments use Crassulacean Acid Metabolism, opening their stomata at night to minimize water loss.
• Silicon accumulation: Grasses actively accumulate silicon in their tissues, which provides structural support, resistance to herbivores, and protection against fungal pathogens.
• Rapid growth response: Many grasses can quickly mobilize resources from rhizomes or crown tissue to produce new growth after defoliation by grazing or fire.