Plant stem
From Free net encyclopedia
A stem is the above ground axis of a vascular plant. The young stem develops in the germinating seedling from embryonic tissue known as the hypocotyl. Lengthening of the hypocotyl lifts the cotyledon(s), plumule (leaf precursor tissues), and terminal or apical meristem above the ground. The surface cells differentiate and mature into a protective epidermal layer. A few cells interior differentiate as chollenchyma, providing support to the young stem. Clusters of elongated cells appear — these are the provascular strands. The remainder of the stem consists of parenchyma cells: those lying between the epidermis and the provascular strands forming a cortex and those interior to the provascular strands forming a pith. Parenchyma radiating from the pith between the precursors of the vascular bundles are called pith rays. The stem of an adult tree is called a trunk.
These tissues are the early precursors of the mature stem tissues, and define the basic functions of the stem:
- Structural support;
- Growth through increase in diameter (girth) and elongation;
- Transport of fluids between the roots and the leaves.
Modified stems for other functions include:
- Tubers, such as the common potato
- Stolons (commonly seen on the strawberry plant)
- Bulbs store energy in fleshy leaves for the initial spring growth spurt of a plant, for example tulips.
- Corms store energy to allow the plant to have multiple flowering seasons, and can grow new corms out of itself. Taro is an example.
- Thorns are modified stems, and function to prevent the plant from being grazed upon. They grow from the axils of leaves. Spines and prickles however, are not modified stems, but modified leaves.
Monocot stems
Vascular bundles are present throughout the monocot stem, although concentrated towards the outside. This differs from the monocot root that has a ring of vascular bundles and often none in the center. The shoot apex in monocot stems is more elongated. Leaf sheathes grow up around it, protecting it. Monocot leaves have continuous vascular tissue going down from a shoot or stem.This is most clearly evidenced when pulling apart the leaves of corn or grass; the leaf fibers continue down wrapped around the stem to its base. This is true to some extent of almost all monocots, and is one of their traits that is more evolved than the older type, dicots. Monocots rarely produce secondary growth and are therefore seldom woody. The cotyledons are not pushed above ground in monocots as they are in dicots.
Dicot Stems
Dicot stems have a pith in the center with vascular tissues in a distinct ring visible in a cross section. They have secondary growth originating from their lateral or secondary meristems: the vascular cambium and the cork cambium. The vascular cambium grows cells that differentiate into secondary xylem and secondary phloem. Secondary xylem is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in most climates.ar:ساق (نبات) bg:Стъбло cs:Stonek da:Stængel (plantedel) de:Sprossachse es:Tallo eo:Pedunklo fa:ساقه fr:Tige he:גבעול lt:Stiebas mk:Стебло nl:Stengel pl:Łodyga sr:Стабло (биљка) fi:Varsi