7: Stems

Major Functions of Stems:

  • conduct H2O and minerals from roots to leaves

  • provide aerial support for leaves

  • some are photosynthetic

  • conduct nutrients via phloem to roots , flowers, young leaves

  • some modified for storage → potato/ginger

  • some horizontal and run just above or below ground

Some stems are diminished (less prominent) but still serve important functions!

  • carnivorous plants → stems are leaf-anchoring

A. EXTERNAL MORPHOLOGY

Many stems are characterized as being herbaceous or woody.

  • Herbaceous stems generally have not undergone significant secondary growth and are often green, fleshy, and flexible

  • Woody stems have undergone secondary growth and developed a periderm. They are typically not green, are harder or firmer, and less flexible.

Note that some herbaceous stems can become woody over time.

Examples of species that are herbaceous at maturity:

  • monocots: lily, orchid, some grasses, onion

  • dicots: mint, parsley, beans, Coleus, clover

Examples of species that are woody at maturity:

  • monocots: palm, date, coconut

  • dicots: apple, oak, maple, Azalea, pecan, redbud

Stem Structures—Familiarize Yourself With These Structures, Using Supplied Images and Lab Specimens

Node: a point of attachment, typically of a leaf or a flower

internode: the area of the stem between 2 nodes

axillary bud: a structure composed of a dormant apical meristem and scales (modified leaves) covering it. Occurs specifically in the axil, which is the angle between a leaf attachment and the main stem axis.

Terminal bud: a dormant apical meristem covered by scales at the apex or end of the stem or the stem branch

Leaf (petiole) scar: the location where a leaf was once attached. Vascular bundle scars may be visible.

Terminal bud scale scars: a ring-like array of scars indicating the location of a previous terminal bud.

Lenticels: small perforations in the periderm of woody stems through which limited gas exchange may occur.

B. Developmental Pattern or Arrangement of Leaves on Stems

Opposite (2 leaves per node): Dogwood, Maple, Ash

Alternate (1 leaf per node): Oak, Hickory, Tulip Tree

Whorled (3 or more leaves per node): Catalpa, Oleander

C. Anatomical Differences Between Monocot and Dicot Stems

Monocot:

  • vascular tissue arrangement: scattered

  • presence of pith: no

  • cortex cells: yes

Dicot:

  • vascular tissue arrangement: in a ring

  • presence of pith: yes

  • cortex cells: yes

What Exactly Are The Differences Between Potatoes, Sweet Potatoes, and Yams?

Potato

  • Solanum sp.

  • Nightshade family

  • Solanaceae

  • Stem

  • Dicot

  • Temperate climate

Sweet Potato

  • Ipomoea sp.

  • Morning glory family

  • Convolvulaceae

  • Root

  • Dicot

  • Subtropical

Yam

  • Dioscorea sp.

  • Yam family

  • Dioscoreaceae

  • Stem

  • Monocot

  • Tropical

Nonbotanical factoids:

  • the “candied yams” you eat are most likely sweet potatoes

  • NC is the #1 sweet potato producer in the USA

Questions you should think about:

What are the anatomical patterns you would expect to find in the roots and stems of these three plants?

Could you distinguish which was a root and which was a stem based on external features?

How could you distingush a monocot stem from a dicot stem?