Stems

 

The autotrophic lifestyle of plants requires that they expose a large surface area to the environment. The most conspicuous surface area is the plant's shoot, which is an integrated system of stems and leaves. Stems support leaves (the "hat racks" upon which plants hang their solar collecting leaves).

 

Stems and their function

A stem is a collection of integrated tissues arranged as nodes and internodes.

††††††††††† Nodes - regions where leaves attach to stems.

††††††††††† Internodes - parts of the stem between successive nodes.

 

 

A cross section of a dicot stem would reveal:

††††††††††††††††††††††† a. An epidermis to the outside (covered by a cuticle, a waxy layer that reduces water loss from the surface).

††††††††††††††††††††††† b. Inside the epidermis is the cortex, a layer several cells thick that is part of the plantís ground tissue system, containing parenchyma, collenchyma, and sclerenchyma cells. From these cells we can see the stem provides 3 functions: photosynthesis, storage, and support.

††††††††††††††††††††††† c. The vascular tissue provides conductance and support. In a cross section it appears as vascular bundles arranged in a circle, and contains both xylem and phloem tissue.Sandwiched between the xylem and phloem is the vascular cambium, a lateral meristem that is responsible for secondary growth.

††††††††††††††††††††††† d. At the center of the stem is pith, a tissue composed of large, thin-walled parenchyma cells that function primarily for storage.

††††††††††††††††††††††† e. The central cylinder of the stem, which includes the xylem, phloem, and pith, is referred to as the stele.

 

Although herbaceous stems all have the same basic tissues, the arrangement of tissues varies considerably.

a.       Herbaceous dicot stems have vascular bundles arranged in a circle (ring) in x-section, and have a distinct cortex and pith.

b.      Herbaceous monocot stems have scattered vascular bundles and ground tissue instead of a distinct cortex and pith.

 

Secondary growth occurs in woody dicots (and some monocots)

a.       The vascular cambium occurs between the primary xylem and primary phloem.

b.      It produces secondary xylem (wood) to the inside and secondary phloem to the outside.

c.       Secondary xylem conducts water and dissolved minerals in the woody stem.

d.      Secondary phloem conducts dissolved carbohydrates in the woody stem.

e.       The cork cambium (phellogen) arises near the stemís surface. It produces cork parenchyma (phelloderm) to the inside, and cork cells (phellem) to the outside.

f.        Cork parenchyma serves mainly for storage in the woody stem.

g.       Cork cells are the functional replacement for the epidermis in secondary growth.

h.       Rays are chains of parenchyma that radiate out from the center of the stem and form pathways for the lateral transport of materials between secondary xylem and phloem.

 

Annual rings, which make concentric circles in wood cut in x-section, are the result of differences in cell size and cell wall thickness between secondary xylem formed at the end of one yearís growth (late summerwood), and that formed at the beginning of the next yearís growth (springwood).

Heartwood, the older, darker-colored wood in the center of a tree, no longer functions in conduction. It is a reservoir of secondary compounds, like tannins.

Sapwood is the younger, lighter-colored wood closest to the bark. Sapwood conducts water and dissolved minerals up the shoot.

 

Functions of stems

1.      Stems support leaves, the solar collector of plants. Turgor pressure in stems provides a hydrostatic skeleton that supports the young plant. Leaves are also supported by a stem's internal skeleton of collenchyma and sclerenchyma.

2.      Stems produce carbohydrates. Stems of some plants, like Parkinsonia (Palo verde) are green and photosynthetic. Altho photosynthesis in stems are insignificant in comparison to leaves, in plants such as cacti it accounts for most of the plant's carbon fixation.

3.      Stems store materials. Parenchyma cells in stems store large amounts of starch and water. E.g. water accounts for as much as 98% of the weight of many cactus stems.

4.      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 the water and dissolved nutrients of the soil.

5.      Stems lift leaves high off the ground to maximize solar interception.

 

Axillary buds and branching

1.      Early in the development of a leaf, a small island of meristematic cells forms in the axil where a leaf attaches to the stem. These cells rapidly form an axillary bud, which typically undergoes a dormant period controlled by hormones made by the shoot apex.

2.      In most plants, axillary buds near the shoot apex remain dormant, while those progressively further away from the tip start to grow.

3.      This dominating effect of the shoot apex on growth of axillary buds is called apical dominance, and strongly influences the symmetry of the shoot.

4.      Apical dominance is strong in plants such as pine and spruce, and accounts for their tiered, Christmas-tree shape.

5.      Conversely, plants with weak apical dominance have a shrublike appearance.

6.      Axillary buds are important because they are a shoot's insurance policy: they are inactive (genetically resting) cells that can form a branch or flower. As such, axillary buds can replace or modify the roles of the shoot apical meristem.

 

Modified, and above-ground stems

Plants often modify their stems for special functions.

1.      Stolons or runners - horizontally oriented stems that grow along the soil surface. Their function is, primarily, vegetative reproduction.

a.      Buds at nodes of strawberry stolons produce shoots and roots that eventually form new plants. Other plants with stolons include the Boston fern (Nephrolepis exaltata), Bermuda grass (Cynodon dactylon), spider plant (Chlorophytum spp.), and crab grass (Digitaria sanguinalis).

2.      Tendrils or twining shoots -modified leaf or stem of a slender strand of tissue. Tendrils coil around objects providing support for the plant. E.g. morning glory and sweet potato. The tips of tendrils, such as Virginia creeper (Parthenocissus), have adhesive pads that stick to nearby objects.

3.      Thorns - modified stems that protect plants from grazing animals, intense heat and insolation. E.g. honey locust (Gleditsia triacanthos), firethorn (Pyracantha), bougainvillea (Bougainvillea spectabilis).

4.      Succulent stems - plants such as cacti have a low surface-to-volume ratio. Succulent stems store large amounts of water and are common in plants growing in deserts.