Structure and types of stems

 There are basically two types of stems - herbaceous stems and woody stems. Herbaceous plants have soft, green, juicy stems that are called herbaceous stems. They have a life span of one to two years. The cells of apical meristems are the source of all the tissues of herbaceous plants. Herbaceous stems are produced by primary growth. Examples of herbaceous dicot include sunflower, buttercup, and alfalfa. The stems of a plant have several functions. Vascular tissue runs through the stem, transporting water, food, and minerals between the roots and the leaves. Some underground stems, such as the white potato tuber, are specialized for food storage.

The cactus stems are modified for storage of water and photosynthesis. The stems of strawberry plants have stems running along the surface of the ground and develop independent plants.

The shoot consists of stem and leaves. The stem serves as a framework for the positioning of the leaves, where most photosynthesis takes place. The arrangement, size, and other characteristics of the leaves are very important in the production of food in plants. Flowers, fruits and seeds are also formed on the shoot.

1.      Internal structure of woody stems

Woody plants have woody stems that are made up of thick, tough tissue that you know as wood. There life span is usually more than two years.

All woody plants are dicots. The stems are tough because of the large amount of xylem that are kept on being added to the thickness of the stem. As shown below, round layers of wood increase the thickness of the stem as xylem builds on the inside of the vascular cambium. The phloem produced by the vascular cambium, does not build up. What happens is that its older layers break off and become new phloem.

1.      Internal structure of herbaceous stems

The outermost tissue is the epidermis and is covered with waxy cuticle to prevent water loss. The vascular tissue is found in bundles that are arranged in a ring (dicots) or scattered (monocots). The central region of the stem is called the pith. Pith is made of parenchyma cells that store food.

Internal structure of herbaceous dicot stems

The growth of new xylem during each growing season results in the formation of annual rings as shown below. Counting the rings can determine the age of a dicot plant. Growth each year is represented by an annual ring.

In young woody dicots, the centre of the stem is filled with pith. The cortex layer is inside the epidermis. For older woody stems, the cells of the pith die, and the cortex is replaced by phloem from the vascular cambium. The xylem lies next to the cambium.

The protective tissue, the bark, is the outermost layer of a woody stem. Bark is made of phloem, cork cambium and cork cells. The cork cells are made by the cork cambium. The inner, younger part of the bark is alive, but outer older part is dead tissue. The older outer bark cracks and comes off as new bark develops.

 

Structure of the protective tissue

2.      External structure of woody stems

At the tip of the plant is the terminal bud which is made of apical meristems enclosed by overlapping protective scales called bud scales. Within the leaf scars are dots called vascular bundle scars. These are the areas at which vascular bundles consisting of xylem and phloem passed from the stem into the leaf.

The axillary or lateral bud is found each leaf scar. These are found above the area where a leaf is or was attached to the stem. These buds may grow into new branches, or may remain small and dormant. Nodes are points along the stem where leaves and literal buds form. The space between two nodes is called an internode. Lenticels are holes that pass through the cork tissue which allows the exchange of oxygen and carbon dioxide between the atmosphere and the internal tissues.