This organ is responsible for the aboveground structure of the plant, and is
involved in both structural support and vascular transport.Recall the general external anatomy of a stem:
Recall the locations of the apical and lateral meristems.
APICAL meristems (located at the tips of roots and shoots) give rise
to three PRIMARY MERISTEMS (protoderm, ground meristem, and procambium).
And also recall:
- ground meristem - develops into ground tissues
- procambium - develops into vascular tissues and the vascular cambium
- protoderm - develops into the dermal system
A cross section of a generalized, herbaceous dicot stem appears on the left:
Note that young, herbaceous stems may have stomates for gas exchange, though the leaf is the main site of gas exchange, with many more stomates than the stem.
Secondary Growth in Stems
LATERAL MERSTEMS are cylindrical, secondary meristems in both stem and root that give rise to either vascular tissue or secondary dermal tissues. They are the
- vascular cambium - located between xylem and phloem
- cork cambium - located between phloem and bark
Recall the progression of secondary growth in the two lateral
determinate growth: growth that occurs during a finite
juvenile phase, and then stops.
indeterminate growth: growth that occurs throughout
the life of the organism.
annual plant: lives for about a year, flowers and dies
perennial plant: lives for more than one year
Annual plants lack secondary growth, and remain herbaceous throughout their short lives. Many perennials do not develop true wood, though they may become somewhat "woody" as their older tissues lignify and become more structurally supported with cellulose, resins, and other substances.
Still others may undergo extensive tissue growth via the vascular cambium. Only these plants produce what is known as true, botanical WOOD.
Stems and Wood
All plants begin their development as HERBACEOUS (i.e., non-woody)
organisms. True WOODY plants eventually develop
Shown diagramatically, the tissue layers are arranged like so:
heartwood: dead center of the woody stem in which conducting elements
of xylem are clogged with tannins and resin, and no longer function to
sapwood: external ring of xylem still conducting fluids
springwood: large-lumen xylem formed in spring
summerwood: small-lumen xylem formed in summer/late autumn, just
The wood of many species may contain species-specific aromatic compounds. Consider:
Why put those in the wood, of all places?
What side effects might this have on humans who use wood?
- wood turning
- wood shavings used for litters
- aromatic wood insect repellants
No tour of stems would be complete without a brief mention of the highly derived stems of monocot anthophytes. A cross section is shown on the right of the diagram we saw above:
Note the absence of concentric rings of vascular tissue. Instead, xylem and phloem are both distributed throughout the pith of the stem in discrete vascular bundles.
The bundle on the left--that of a typical dicot--shows that the basic structures inside a primary vascular bundle are similar in these two groups. The most important difference is their arrangement in the stem ground tissue.
In dicots, the bundles form a ring around the central pith. In monocots, the bundles are scattered randomly throughout the ground tissue (sometimes called pith).
Monocots (with the exception of the most primitive species, the Joshua Tree (Yucca brevifolia Engelm. var. brevifolia; Family Liliaceae) ) lack a vascular cambium or cork cambium. Therefore, these monocots do not produce true, botanical wood (concentric rings of xylem), although they may be very "woody" in some cases (e.g., palms, large bamboos).
Stem Gas Exchange
How does an enormous tree get enough oxygen for its needs, if all its skin is woody?
In some species, gas exchange pores called lenticels. These are "spongy" regions found on the bark of stems (and sometimes aerial roots) of woody vascular plants. These areas, which form around what used to be groups of stomates, allow gas exchange between internal tissues and atmosphere across the periderm, which is otherwise impermeable to gases.
The cork cells of the lenticel, unlike those forming the rest of the cork, have minute air spaces between them that allow oxygen and other gases to enter and leave the plant tissues.
You've probably seen lenticels, but didn't realize you were seeing them.
Even a few fruits have lenticels; these are often accessory fruits (i.e., fruits developed from more than just an ovulary). In the case of the apple or pear, the skin is what used to be the flower receptacle: a modified stem.
Stems may be highly derived in form and function.
rhizome - underground stem. Typical of ferns
and some other plants
tuber - underground storage stem is a modified rhizome.
tendril - typical of climbing vines, these respond
to touch and grow around supporting items.
stolon - above-ground propagative root (e.g. strawberry; spider plant) that produces new plantlets asexually. (A form of cloning) These are often called "runners". (Other examples: strawberries, many grasses)
bulb - an underground stem consisting of a dense basal plate (a shortened stem axis), and growing point or shoot primordium, enclosed by thick, fleshy modified leaves.
corm - a solid modified stem consisting of a swollen base of a stem axis enclosed by dry, scalelike leaves. These usually have fibrous and contractile roots.
pseudobulb - unique to orchids, these are thickened, bulblike stems that store both water and nutrients.
cladophyll - flattened stem that serves
the photosynthetic function of a leaf. Another example: various species of
cactus). From the Greek clad, meaning "branch" and phyll, meaning "leaf".
thorn - Although not all spines you see on a plant are modified stems, thorns are shortened stems modified to form sharp, protective spikes. (Other types of spines can be extensions of the periderm, or even modified leaves, as in cactus spines. They are not always modified stems.)