Organs

Plants consist of several organs, which in their turn are composed of tissues. Broadly, vegetative organs support plant growth, and reproductive organs enable sexual reproduction. The three main types of vegetative organ are the root, stem and leaf. Roots typically occur underground, and extract moisture and nutrients from the soil, though there are many examples of plants with aerial roots. The stem and leaves together comprise the shoot (Fig. 1.1). Stems occur both above and below ground. Some stems are modified into underground perennating or storage organs such as corms or rhizomes. Leaves typically occur above ground level, though some underground stems possess reduced scale leaves, and underground bulbs possess swollen leaves or leaf bases.

Primary organs and tissues develop initially from the shoot and root apical meristems and from cell divisions in meristems closely adjacent to them, such as the primary thickening meristem. Secondary tissues such as secondary xylem (wood) develop from lateral meristems such as the vascular cambium. Organs such as adventitious roots develop from differentiated cells that have retained meristematic capacity. At the onset of flowering, the shoot apical meristem undergoes structural modification from a vegetative to a reproductive apex and subsequently produces flowers (chapter 5). Flowers are borne on an inflorescence, either in groups or as solitary structures. A group of inflorescences borne on a single plant is termed a synflorescence (Fig. 1.2).