|Themes > Science > Botanical Sciences > Plants and their Structure > Stem and its Shoot|
Support of Leaf Display The stem must
be strong enough and flexible enough to position the leaves in the
sunlight to optimize the photosynthesis of the plant.
Support of Flower Display The stem must put the flowers into a correct (usually visible) position to attract the appropriate pollinators. The stem must be able to support the weight of some perching pollinators. In the case of wind pollination, the flower must be positioned properly for male parts to be upwind from the female parts.
Support of Fruit Display A plant must be able to support its load of developing fruit. For wind-dispersed fruits and seeds, getting the fruits up to the wind exposure is critical. For animal-dispersed fruits and seeds, the stem must position the fruit for best display or best attachment to the animal parts.
Conduct water and minerals up from soil The flow of xylem continues from root to the leaves via the stem. The stem must not only conduct minerals and water up through the plant, but it must distribute it to all elements in the shoot system (leaves, flowers, fruits, etc.).
Conduct water and nutrients Sugars and amino acids from photosynthesis in the leaf are conducted by the phloem in the stem to the rest of the plant. This flow will be upwards from leaf to apical bud, to flower, or to fruit as well as downwards from leaf to root.
Photosynthesis In some species, such as cacti, the stem is the primary site of photosynthesis. In cacti, the leaves are converted to non-green spines, and the branches to non-green areoles.
Storage of water, etc In the stems of cacti, in underground stems such as in potato, and in the swollen hypocotyl of radish, water can be stored. Of course this water needs to be protected from herbivory in desert situation.
Defense Desert plants have evolved spines (from leaves) and glochidia (from lateral buds) to mechanically protect the stems from herbivory. The stem must position these appendages and support them sufficiently for them to function. In other species, such as Lophophora williamsii, the peyote cactus, the stem posesses mescaline or other potent chemistry to deter and/or kill herbivores.
Anchorage Especially in vines, stems provide anchorage functions. This anchorage can be accomplished by: twining as in morning glory, by the formation of adventitious roots as in poison ivy, or by the formation of "suction cups" as in Virginia creeper.
The Origin of the Shoot
The stem and its shoot system originate at the shoot apical meristem. A photomicrograph is shown below of the primary meristems: protoderm, ground meristem, and provascular. You will notice that the zones of division and elongation are interrupted by nodes for leaf attachment. The shoot apical meristem also has appendages: leaf primordia. There is no cap on the shoot tip; it grows into the air and does not need protection from abrasion. However the leaf primordia tend to arch over the zone of cell division to protect this tender meristematic tissue from herbivory and desiccation.
This is a longitudinal section of a shoot tip:
This view of a shoot apex is as diagrammed below:
The Mature Dicot StemThe protoderm matures to become the epidermal system. The ground meristem matures into cortex (just under the epidermis) and pith (just inside a ring of vascular bundles).
Each vascular bundle consists of four layers.
Toward the cortex the bundle may show a layer of fibers. These are sometimes called phloem fibers and the individual cells are highly elongate having expanded by intrusive growth. Some individual fiber cells can be in the range of meters in length. These fibers have been removed from stems, spun into thread, and woven into fabric. The fibers of flax were treated this way right here in Willimantic. At one point our little town was the world's leading supplier of linen!
Moving interior to the fibers, we find the functional primary phloem. Here sieve tube elements conduct water and sugars and amino acids from the leaves to the rest of the plant. The sieve tube cells are alive, but lack nuclei and other organelles. They are comparable to red blood cells in humans. However, unlike RBCs, phloem sieve tube elements can live for years. This ability is provided by adjacent companion cells. The companion cells have complex cytoplasm with mature organelles and provide the metabolic and homeostatic needs of the sieve tube elements through plasmodesmata...cytoplasmic strands that connect between these two types of cells. The presence of two cell types with vastly different appearance gives the functional phloem area a superficially "messy" look. A closer examination reveals some regular patterns in the distribution of the various cell types in phloem tissue.
The next layer toward the pith is the vascular cambium (aka cambium). These cells are arranged in regular rows and columns like an accountants spreadsheet. These cells are meristematic and divide mitotically to produce additional secondary xylem and secondary phloem in woody plants. This layer may be absent in some non-woody species...such as grasses.
The innermost layer of the vascular bundle is the primary xylem. This tissue conducts water and soil minerals up the stem from the roots. Like human skin cells, most of these xylem elements are dead at functional maturity. The tracheids and possibly vessels of the plant stem serve as plumbing. They lack cytoplasm altogether at maturity...the last act of the cytoplasm in the developing tracheary element is to digest its own cytoplasm and sometimes also its end walls to form the pipes through which the water and minerals are conducted.
If there is time in class I will help you learn about xylem maturation (exarch in roots, endarch in stems) and bundle types (bicollateral in Cucurbita, colateral in Helianthus). And we may also deal with hollow stems too.
Monocot Stem AnatomyMonocots such as grasses are put together in a similar fashion. The epidermis serves window and gas exchange functions, the ground parenchyma may perform photosynthesis and/or storage functions. The stem of sugar cane is famous in that regard. The vascular bundles are typically NOT arranged in a single circlar array. Rather a range of concentric rings of various diameters are found. Careless authors will call the arrangement "scattered;" that is a pet peeve of mine. Just because we humans are to simple minded to look more carefully to see the pattern, we lazily call it "scattered" as if they are whimsically arranged. Like Tevya's one long staircase just going up, one even longer going down, and one more leading nowhere just for show, a scattered array would make no sense at all. There is order in that COMPLEX array, OK?!
Here is a view of a monocot stem cross section: