he organs of a plant — the leaves, stem, and roots — are made up of tissues containing cells with specific functions and shapes. All organs are originally derived from apical meristems. The leaves and stems are collectively known as the shoot system. Leaves, typically divided into a petiole and blade, generally are the primary photosynthetic organs of the plant. Leaves are attached to the stem at a node. The stem supports the plant, and houses transport structures.
In most plants, the below-ground parts are composed of roots, and are collectively called the root system, but aerial roots occur in some plants, such as Hedera (Ivy) and epiphytic orchids; and adventitious roots arise from stems, branches, leaves, and old woody roots. There are exceptions where plants have above-ground modified roots, and below-ground modified stems. Roots anchor the plant and absorb water and minerals through their root hairs. The development of each system is controlled largely by genes but also by the environment.
Most of the plant is ground tissue, which is made of parenchyma, collenchyma, and sclerenchyma cells. These cells also compose part of the dermal and vascular systems. Ground tissue performs metabolic, storage, and support functions. Parenchyma cells store and regulate ions, water, and waste products. They also produce and store starch. Collenchyma cells, with their thickened primary walls, provide support and flexibility to growing regions of the plant, particularly in the stems and leaves. Sclerenchyma cells have a secondary cell wall, are typically lignified, and are the principle structural support in tissue that has ceased to grow in length. For example, these cells provide the outer covering for nuts and fruit seeds and provide structural support to prevent stems from collapsing.