Stems as sources of taxonomic characters
Stems – characteristics of the vascular tissue:
- Monocots vs Eudicots. Eudicots have a ring of vascular bundles when young (xylem inside, phloem outside). Monocots have vascular bundles scattered throughout the stem.
- Some orders of Eudicots have vascular bundles with internal phloem. Each vascular bundle has xylem in the middle and phloem outside and inside. This feature distinguishes Solanales (Solanaceae and Convolulaceae) from Scrophulariales and other Asterids.
Stems - duration and life form:
- Duration: annual (life cycle in one season), biennial ( two growing seasons), herbaceous perennial (a “non-woody” plant - in cold climates the stems often die back to the ground during winter), woody perennial (perennial with stems that persist and become more woody with age)
- Main growth forms:
tree (single trunk),
shrub (several woody stems),
herb (“nonwoody” plant, but - NOTE - even Arabidopsis may have a very small amount of wood when old!),
vine or liana (climbing plant),
epiphyte ( growing upon another plant)
Some taxonomically significant modes of growth:
- Acaulescent - above ground stem absent or inconspicuous (a rosette herb).
- Decumbent - stems lying flat on the ground but turning up at the ends.
- Prostrate - stems growing flat on the ground
- Reproductive stems: stolon ("runners" of a strawberry), bulbil (small bulbs, often borne in leaf axils (Lilium) or replacing flowers (garlic)
- Underground stems: rhizome (horizontal, underground stem with reduced scale-like leaves - Iris), corm (a short, upright, underground stem, covered with thin dry leaves - Crocus), bulb (as corm but with thick fleshy, non-photosynthetic storage leaves - onion), tuber (swollen tip of underground stem as in potato, BUT not sweet potatoes - tuberous roots).
- Water storage: succulent stems (cacti)
- Defence: there are three main types of defensive armature produced by plants, thorns, prickles and spines. Two of these are produced from stems as follows
(1) thorns are modified stems: usually reduced, sharp pointed, hard stems, as in hawthorn (Crataegus)
(2) prickles are stem dermal products: sharp pointed extensions of the epidermis and cortex of a stem - as in a rose or blackberry (Rosa and Rubus).
(3) spines are modified leaves, as in the Cactaceae.