Stem is a supportive structure of a plant. It is an essential part of the structural integrity of all the plants as the stem assist in the proper positioning and elevation of leaves, fruits, andflowers.Because of the stem, the leaves and branches are arranged in an orderly manner. Stem holds the twigs and leaves in place. The Xylem and Phloem found in the plant stem help in the conduction water across the plant. In some plants, stems store food, water, and minerals. The meristems help in the formation of new living tissues.
Structure of stem
Stem is divided into 3 major parts Epidermis, Ground tissue and the Cortex. The epidermishas a single layer of living stem cells. Stem cells are cutinized with the help of cuticle on their surface. Cutinization prevents water loss. Stomata are rarely found in the epidermis but the chloroplasts are totally absent. With the help of epidermis, stem assist in the primary and secondary growth of the plant. Below the epidermis, there is a ground tissue. Ground tissues divide itself into 2 portions- the central portion and the cortex. Cortex lies between the vascular tissue and the epidermis. Cortex is connected with the pith by the help of medullary. Cortex has a series of layers as explained below.
Structure of cortex
The hypodermis is the outermost layer of the cortex made up of four to five thick layers of living cells, called collenchymatous cells. Below the hypodermis, the general cortex is present. The general cortex consists of thin-walled parenchymatous cells composing of many intercellular spaces. Those cells that have chloroplasts are known as chlorenchyma. The endodermis is the innermost layer of the cortex made up of barrel-shaped cells arranged in a single row with no intercellular spaces in between. Endodermal cells help in storing the starch grains hence it is called the starch sheath. Endodermis has distinctly visible endodermal cells.
The vascular tissues are made up of complex xylem and phloem. The xylem and phloem carry water and nutrients up and down through the length of the stem. The arrangement of xylem and phloem in the form of distinct strands is termed as vascular bundles.Cambium is a strip of thin-walled cells found between the xylem and phloem. Cambium is made up of merismatic cells responsible for the secondary growth in a plant. Cambium is exclusively found in dicot plants but absent in the monocots. In a typical plant, the Vascular bundles are arranged in collateral and bicollateral manner. Collateral is when xylem is placed towards the inner side and phloem is located outside side of the vascular bundle. Bicollateral arrangement is characterized by the presence of phloem on both sides of xylem.
Functions of Stem
Stem assist in supporting and holding the leaves, flowers, and fruits.
It provides an opportunity for leaves to be arranged in a way that they can easily receive direct sunlight necessary for photosynthesis.
Stem enhances gaseous exchange by allowing the holding the leaves in a manner that exposes the leaves to the external environment.
The xylem and phloem in the vascular bundles transport water and minerals across the membranes.