Themes > Science > Botanical Sciences > Plants and their Structure > Stems

A stem is a collection of integrated tissues arranged as nodes and internodes. Nodes are locations where leaves attach to stems, and internodes are the parts of stems between nodes.

Nodes and internodes perform several important functions: 

• Support leaves-Turgor pressure in stems provides a hydrostatic skeleton that supports young plants. Leaves are also supported by a stem's internal structure of collenchyma and sclerenchyma.
• Produce carbohydrates-Stems of plants are green and photosynthetic. Although photosynthesis in stems is usually not significant compared to leaves, in some plants such as cacti it accounts for most of the plants carbon fixation.
• Store materials-Parenchyma cells in stems store large amounts of starch and water. Water accounts for as much as 98% of the weight of many cactus stems.
• Transport water and solutes between roots and leaves-The vascular system of stems maintains an aquatic environment in leaves and transports sugars and other solutes between leaves and roots. Stems link leaves with he water and dissolved nutrients of the soil.

Control of Stem Growth:

The growth of stems is controlled by many factors, leaves being the most important. Leaf primordial control the differentiation of procambium in stems. There is no procambium above the youngest primordium, and vascular tissues in developing leaves along and connect with mature vascular tissues of the stem. Light also controls stem growth.

Stems elongate in subapical regions in response to auxin and gibberellins. These hormones elongate stems by stimulating cellular elongation and division. In a lot of plants, elongation occurs throughout the internode; while in some others it continues as a wave originating at the base of the internode.

Grasses such as bamboo elongate at meristems intercalated between mature tissues at the bases of their nodes and leaf sheaths. These meristems are called intercalary meristems. These remain important because they reform the stem or leaf of a grass when its tip is torn off by an animal or lawn mower. Intercalary meristems can produce high rates of growth in many plants. They also have a direct impact on us-no matter how many afternoons we spend behind the lawn mower, we are assured that the grass blades will grow back quickly because of intercalary meristems.

Plants whose stems do not elongate are called rosette plants; these plants have short internodes with tightly packed leaves.The stems of rosette plants are short and made almost entirely of overlapping leaf bases. Most rosettes fail to elongate because they do not make enough physiologically active gibberellins. Treating rosettes with gibberellins usually causes internodal elongation.

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Structure of Stems:

Stems are composed of three structures: 

• Epidermal tissue-Stems are encased by a transparent epidermis which is usually about one cell thick and often has trichomes. The trichomes serve various purposes; some plants secrete juices to attract insects, which others have hoo-shaped trichomes which often entangle the insects and keep them from feeding while they struggle to free themselves.
• Vascular tissue-As I mentioned before, there is no procambium above the youngest leaf primordium, and removing a plant's leaf primordia stops vascular differentiation. So, substances coming from young leaves control the differentiation of procambium and vascular tissues in stems. Xylem and phloem in stems occurs in vascular bundles. Phloem forms before the xylem and it differentiates on teh outside of the bundle. Xylem forms on the inside of the bundle. Vascular bundles are often enclosed by sclerenchyma fibers that differentiate after the internode has finished elongating. But, there is a layer of cells between the xylem and phloem, which remains meristematic. In woody plants and some herbaceous dicots, this layer of cells later becomes part of the vascular cambium, and produces secondary growth. The absence of a vascular cambium in monocots, is an important feature. Vascular bundles in stems are arranged differently in different plants.
• Ground tissue-Between the epidermis and the ring of vascular tissue in dicots is the cortex. Most cells of the cortex are parenchyma. Cortical cells are photosynthetic in plants and often store starch. In dicots, the ground tissue with the parenchyma cells in the center of the stem is specialized for storage and is called pith. Pith cells are often lignified, arranged loosely, and the pith may contain secretory structures such as laticifers. Because monocots have vascular bundles throughout their ground tissue, their stems do not have a discernible pith; the parenchyma cells in monocot stems are referred to simply as ground tissue.

Axillary Buds and Branching:

Early in leaf development, a small island of meristematic cells forms in the axil where a leaf attaches to the stem. These cells quickly form an axillary bud, that undergoes a dormant period controlled by hormones made by teh shoot apex. In most plants, axillary buds near the shoot apex stay dormant, while those farther away from the tip start to grow. This dominating effect of the shoot apex on growth of axillary buds is called apical dominance and influences the symmetry of the shoot. Plants with weak apical dominance have a shrub shape. Axillary buds are important because they are a shoot's insurance policy: they are inactive cells that can form a branch or flower.

Modified Stems:

Plants often modify their stems for special functions. These modifications can have unusual and even bizarre shapes, and can grow above as well as below the ground. 

Modified stems that grow above ground: 

• Stolons or runners-are horizontally oriented stems that grow along the soil surface. Their function is vegetative production.
• Tendrils and twining shoots-coil around objects and help support the plant.
• Searcher shoots-are stems with long internodes that move in circles through the air seeking a support.
• Cladodes or cladophylls-are flat, leaflike stems modified for photosynthesis.
• Thorns-are modified stems that protect plants from grazing animals.
• Short and long shoots-long shoots have long internodes, while short shoots are rosettes and have short internodes.
• Succulent stems-of plants have a low surface to volume ration and store large amounts of water and are common in desert plants.

Modified stems that grow below ground: 

• Bulbs-are rosette stems surrounded by fleshy leaves that store nutrients. When the nutrients are removed, the leaves collapse.
• Rhizomes-are underground stems that grow near the soil surface. They typically have short internodes and scale leaves, and produce roots along their lower surface. They store food for renewing growth of the shoot after periods of stress.
• Corms-are stubby, vertically oriented stems that grow underground, have only a few thin leaves, and store nutrients.
• Tubers-are swollen regions of stems that store food for subsequent growth.

By: Becky Earley
Information provided by: http://wwwfac.wmdc.edu