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Photo shows rough, white ovals embedded in a smooth, reddish brown woody tree trunk. Where the ovals are, it appears as if the bark has been scraped away.
Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere. (credit: Roger Griffith)

Annual rings

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem ( [link] ). An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.

 Photo shows a cross section of a large tree trunk with many rings projecting outward from the center.
The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate—note how the rings vary in thickness. (credit: Adrian Pingstone)

Stem modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment ( [link] ). A rhizome    is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato ( Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb    , which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

 Photos show six types modified stems: (a) Lumpy white ginger rhizomes are connected together. A green shoot projects from one end. (b) The carrion flower corm is conical-shaped, with white roots spreading from the bottom of the cone, just above the dirt. (c) Two grass plants are connected by a thick, brown stem. (d) Strawberry plants are connected together by a red runner. (e) The part of the potato plant that humans consume is a tuber. (f) The part of the onion plant that humans consume is a bulb.
Stem modifications enable plants to thrive in a variety of environments. Shown are (a) ginger ( Zingiber officinale ) rhizomes, (b) a carrion flower ( Amorphophallus titanum ) corm (c) Rhodes grass ( Chloris gayana ) stolons, (d) strawberry ( Fragaria ananassa ) runners, (e) potato ( Solanum tuberosum ) tubers, and (f) red onion ( Allium ) bulbs. (credit a: modification of work by Maja Dumat; credit c: modification of work by Harry Rose; credit d: modification of work by Rebecca Siegel; credit e: modification of work by Scott Bauer, USDA ARS; credit f: modification of work by Stephen Ausmus, USDA ARS)

Watch botanist Wendy Hodgson, of Desert Botanical Garden in Phoenix, Arizona, explain how agave plants were cultivated for food hundreds of years ago in the Arizona desert in this video: Finding the Roots of an Ancient Crop.

Some aerial modifications of stems are tendrils and thorns ( [link] ). Tendrils are slender, twining strands that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified branches appearing as sharp outgrowths that protect the plant; common examples include roses, Osage orange and devil’s walking stick.

 Photo shows (a) a plant clinging to a stick by wormlike tendrils and (b) two large, red thorns on a red stem.
Found in southeastern United States, (a) buckwheat vine ( Brunnichia ovata ) is a weedy plant that climbs with the aid of tendrils. This one is shown climbing up a wooden stake. (b) Thorns are modified branches. (credit a: modification of work by Christopher Meloche, USDA ARS; credit b: modification of work by “macrophile”/Flickr)

Section summary

The stem of a plant bears the leaves, flowers, and fruits. Stems are characterized by the presence of nodes (the points of attachment for leaves or branches) and internodes (regions between nodes).

Plant organs are made up of simple and complex tissues. The stem has three tissue systems: dermal, vascular, and ground tissue. Dermal tissue is the outer covering of the plant. It contains epidermal cells, stomata, guard cells, and trichomes. Vascular tissue is made up of xylem and phloem tissues and conducts water, minerals, and photosynthetic products. Ground tissue is responsible for photosynthesis and support and is composed of parenchyma, collenchyma, and sclerenchyma cells.

Primary growth occurs at the tips of roots and shoots, causing an increase in length. Woody plants may also exhibit secondary growth, or increase in thickness. In woody plants, especially trees, annual rings may form as growth slows at the end of each season. Some plant species have modified stems that help to store food, propagate new plants, or discourage predators. Rhizomes, corms, stolons, runners, tubers, bulbs, tendrils, and thorns are examples of modified stems.

Art connections

[link] Which layers of the stem are made of parenchyma cells?

  1. cortex and pith
  2. epidermis
  3. sclerenchyma
  4. epidermis and cortex.

[link] A and B. The cortex, pith, and epidermis are made of parenchyma cells.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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