Stem Structure

The Shoot System
 The shoot system comprises the leaves and stems of plants. Shoots develop from the shoot meristem, which contains the shoot tip meristem, leaf primordia and bud primordia (which are embryonic lateral shoot systems). Shoot meristems are located at the growing tips of stems, and in buds.

Shoot Meristems
 As a shoot grows, buds are laid down by shoot meristem in the axils of leaf primordia. These buds are dormant meristems which are activated at some later time in growth. Stem tissues are produced from the same three derivative meristems as root tissues are:
Protoderm is responsible for Epidermis
Ground Meristem differentiates into ground tissues
Procambium produces the Vascular tissues

The arrangement of derivative meristems is different in the stem from the root meristem. In the shoot meristerm, procambium procambium forms a cylinder of cells with ground meristem both to its interior, and to the exterior, of the procambium cylinder. As expected, the protoderm is the outer layer of derivative meristem. Note that single, central strands of procambium extend out into the leaf primordia, however. These are called leaf traces, and leave a vascular gap in the stem tissue at those points.

The positioning of new leaf primordia, and hence branches, is regulated by the inhibition effect of existing primordia so that leaf and branching patterns generally spiral along the stem.

In this section we will look at the internal and external structure of stems. Both dicot and monocot stem structure will be discussed, as well as the stem structure of conifers. Leaf structure will be discussed in the next section.
Stem Functions
 The Stem is the axis of the shoot system which provides mechanical support.

  • The Stem is the axis of the shoot system which provides mechanical support for and serves as the attachment site for leaves and reproductive shoots.
  • Stems elevates leaves for photosynthesis and position reproductive shoots for optimal access to pollinators and dispersal agents.
  • Stems conduct water and minerals from roots to the leaves and solutes from leaves to storage and use sites.
  • Stems are responsible for the overall growth (height and girth) of the plant from the shoot primary and seconday meristems

    There is a tremendous variation in stem types and stem anatomy, although there are some common features, which we will discuss. Most secondary growth of plant shoot systems (increase in girth) occurs in stems. (Leaves are primary growth structures) Our discussion of stems will include both internal anatomy and external features of primary and secondary growth in stems.

    External Features of Stems

    It is difficult to generalize about the external structure of stems. There are so many variations and modifications of stems from herbaceous to woody stems that it is impossible to state that a stem looks like "this". However, there are a number of easily recognized common features of the twigs (small branches) of any woody stem, so much so, that for deciduous trees, there are identification keys that focus on winter twig and bud patterns.

    Before turning to the features of secondary growth in stems, let's spend a little time identifying the external features of twigs:

    Most twigs have both a terminal (or apical) bud, located at the tip of the twig, and a number of axillary (or lateral) buds, located in the axils of leaves. Buds are dormant, protected by from one to several bud scales (which are modified leaves). Leaves are attached to the stem at nodes; the space along the stem between leaves is called an internode. A leaf scar remains on the stem when leaves dehisce. The pattern of leaf scars is species specific. The vascular bundles of the leaf petiole also leave bundle scars within the leaf scar. Each growing season, the bud scale scars of the terminal bud can be seen on twigs. This is one way to determine the age of a twig. Young branches and stems also have specific markings along the surface bark, called lenticels. These structures function in gas exchange. As secondary growth continues, and branches increase in girth, the expansion of cork and bark eventually replaces the features found on twigs. Older branches take on the bark pattern associated with the specific species.
    Primary Growth of Stems
     There are two basic primary growth patterns in stems:

    While the arrangement of tissues in stems differs from roots, the tissues are the same. and by now, familiar.

    Dicot Stem Primary Growth Pattern
     Epidermis
     Cortex region
     Ring of discrete vascular bundles
     Primary Phloem toward outside (rarely may have phloem on the inside of the xylem, too)
    When there is secondary growth, there will be a layer of procambium retained between the primary xylem and primary phloem. Such vascular bundles are said to be open vascular bundles , because they can proceed to secondary growth. Dicots which lack secondary growth have closed vascular bundles, and no procambium remains between the primary xylem and primary phloem.

     Pith
     Monocot Stem Variations
     Most monocots are reasonably small, herbaceous plants. The common monocot families are the lily, grass and orchid families. There are some notable exceptions, however, such as the palms. Most monocots have no secondary growth, even those which are perennials.

    Some distinctive monocot stem features:
    Generally no cambium (no increase in girth)
    There may be 2 or 3 layers of sclerenchyma beneath the epidermis layer for strength and support of the stem structure. Some parenchyma cells may also develop thickened walls in monocot stems as they mature.
    Vascular bundles are "scattered" in appearance throughout the ground parenchyma, so there is no distinction between cortex and pith. The parenchyma cells between vascular bundles are just referred to as ground tissue.
    Special Variations of some Monocots

  • Hollow stems
    A central pith cavity develops from cells of the ground tissue which are destroyed during early growth. This does give an effect of a ring of vascular bundles, similar to dicot patterns
  • Intercalary meristems
    Many grasses have meristem layers at the bases of nodes, which provide for non-apical growth and enlargement of cells throughout the plant stems. (Which is why we have to mow lawns).

    Secondary Growth in Stems
     Recall that secondary growth in plants is responsible for the increase in girth or diameter of the plant by the addition of secondary vascular tissue . Although we focus on secondary growth in stems, roots, too, have secondary growth patterns which parallel the secondary growth of stems. Leaves rarely, if ever, have secondary growth.

    Secondary growth has:

  • Great commercial value (wood) for humans
  • Great dimensional value for plant - allows for much greater size
  • Increase in diameter (girth) by the addition of vascular tissue which develops from a vascular cambium (which is produced from procambium)
    Vascular cambium produces secondary xylem to the interior of the cambium layer, and scondary pholem exterior to the cambium layer.
  • Formation of cork, a replacement layer for epidermis, which develops from cells of the outer cortex which dedifferentiate to form a cork cambium.


    Initiation of secondary growth

    Secondary vascular tissue
    Secondary vascular tissue growth is initiated in the primary growth stem, from cambium cells located in open vascular bundles. There are two types of cambium in the early stages of secondary growth:

  • Vascular bundle cambium (or fascicular cambium)
  • Inter-bundle cambium (or inter-fascicular cambium) which grows to close gaps between bundles forming a vascular cylinder

    Once a vascular cylinder is formed, cambium produces secondary xylem toward the interior of the stem and secondary phloem toward the exterior of the stem. Additional cambium cells are found between the xylem and phloem, and also divide "sideways" to maintain a continual cambium cylinder as the diameter of the stem increases. Most cells produced are xylem.

    Secondary dermal tissue is produced from the cork cambium, which produces cork tissue and cork parenchyma cells, called phelloderm. The tissues produced by the cork cambium are collectively called the periderm.

    As the original epidermis and cortex layers are destroyed and sloughed off, they are replaced by cork. Cork tissue interlaces with secondary phloem tissue to form bark. The continued production of new vascular tissue (xylem and phloem) forces the stem to expand outward. Older phloem and cork are eventually sloughed off, and are themselves replaced with more bark. This creates a number of interesting patterns in bark. The bark pattern of a tree is also a species characteristic. In contrast, all of the secondary xylem is retained as the stem expands, forming the wood portion of the stem.

    Tissues in Secondary Growth
     1. Xylem - wood
     Recall that all cells which mature inward from the vascular cambium are xylem tissue. This tissue forms that part of the stem we call wood.

    Wood Orientation
     When wood is observed, many different patterns or grains are possible depending on the orientation (or plane) of wood when it's cut Since we know that cells have three dimensions, the way in which tissue is cut and observed will give a different appearance to the cells. The orientation of the rays are most important in determining grain patterns.

    Transverse or cross section
    See the "ends", or cross sections (short dimension) of vessels, tracheids and fibers
    Radial longitudinal section (quarter-sawed timber)
    Tangential longitudinal section (plain-sawed timber; perpendicular to radius)
    Xylem patterns and wood grain
     The different patterns and growth rates of xylem tissues are responsible for the "grain" or patterns found in wood. The grain appearance will also vary with the orientation or plane of the wood cut (as discussed). Climate also is important to wood grain, and the "growth" rings found in most wood.
    1. Growth Rings:
  • Spring wood ---> Larger vessels, more porous and smaller, fewer rays
  • Summer wood ---> Denser, smaller cells with thicker walls
  • Trees native to tropical regions which have a uniform climate, such as the tropical rain forests, may not have annual growth rings, and have a very uniform grain. Other tropical trees which grow in areas with seasonal climates, do have growth rings, although the seasons may be wet/dry rather than cold/warm

    Note: Deciduous refers to a seasonal loss of all of a tree's leaves. Some trees are evergreen, which means that individual leaves are replaced as they die.

    2. Distribution of and vessel size in wood
  • Ring Porous ---> Transition abrupt between spring and summer wood. Spring vessels are distinctively large and fewer, and the summer vessels are numerous and small.
  • Diffuse Porous ---> Vessels are produced more uniformly in the wood of both spring and summer wood.

    3. Ray patterns
  • Rays are detectable when they are in clumps and that affects the grain.
  • Rays which are a single cell wide are visible only microscopically

    4. Heartwood and Sapwood
  • Heartwood
    Non-functioning older xylem, often darker
    Heartwood can rot away, leaving a hollow core, with living tree around it.
  • Sapwood
    The younger functioning xylem, closer to the cambium layer

    5. Knots
  • Where branches originated when stem was younger. Eventually, growth in girth surrounds the branch, forming the knot.

    Conifer Wood
     Much of the wood used commercially is from conifers. Conifers are often called softwood, which the wood of Flowering Plants is known as hardwood. This has no meaning for the strength of wood....
    Conifers have tracheids and no vessels in their xylem, so the wood patterns appear more uniform than the wood of flowering plants which have the larger diameter vessels, along with the numerous fibers.
    Microscopically, the bordered pits of tracheids are spectacular.
    It is also common for conifers to have resin canals or resin ducts in both wood and bark. The resin ducts are lined with parenchyma cells. and contain pitch.


    2. Bark
     Although the bulk of secondary growth occurs in the xylem (wood), there is a. second major area of secondary growth, the bark . The bark comprises all regions of the secondary growth stem exterior to the cambium. This includes phloem from the vascular cambium, and cork tissues.

  • Cork
     Recall that primary growth produces the epidermis for protection and collenchyma cells for strength. However, as the stem enlarges with secondary growth, the primary tissues can not grow, and must be replaced. These tissues are replaced by a cork cambium (which originates from parenchyma like cells of outer cortex (or inner epidermis) The cork cambium produces cork cells.

    Characteristics of cork

  • Phloem
     Secondary phloem originates from cambium cells which divide and specialize outward. As secondary growth progresses, functioning phloem sieve tubes and companion cells are spread out in patches interspersed with dilated phloem rays (parenchyma cells), and intermixed with cork.
  • Lenticels in Bark
     Since cork is generally impervious, special structures for gas exchange are required for the secondary growth stem living cells. Lenticels are weak "eruptions" of parenchyma cells through which gases can diffuse. Lenticels also contribute to the appearance of bark

    Aging of Bark
     Since volume expands constantly the bark must likewise increase in growth to accommodate the interior expansion. Old bark is continuously being pushed outward and on occasion will be shed from tree by sloughing off. The different ways of sloughing result in unique bark patterns, such as papery, furrowed, or shredded.


    Secondary Growth in Monocots
     Recall that we stated that monocots generally lack secondary growth and most monocots are small and herbaceous (which is consistent with the lack of secondary growth. However there are some significant exceptions to the overall small size of monocots. There are a number of ways monocots increase in girth with little or no secondary growth.

    Option 1.
    Plant increases in diameter as the seedling emerges by a series of short internodes at the base resulting in a broad base with many vascular tissue and much parenchyma.
    The effect of this is
  • Uniform upward diameter
  • Thickened parenchyma cells for support
  • Long lived phloem
  • Large apex (or tip) providing for
  • Leaves with large vascular connections which sheath the stem increasing diameter
    Example = Palms

    Option 2.
     Prop roots develop providing long-term support for the plant
    Example = Pandanus or corn

    Option 3.
     Produce a cambium which produces additional vascular bundles, but not a "wood" This increases volume but not necessarily a "tree-like" organism
    Example = Agave
    Option 4.
     Sheath the stem with giant leaves which have extraordinary vascular tissue (vein) connections. Recall that veins have many sclerenchyma fibers.
    Unfortunately, these plants are quite short-lived, although they achieve big dimensions
    Example = Banana

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