The Shoot System: Primary Stem Structure - 1 Shoot System The shoot system comprises the leaves and stems of plants. Leaves are located at nodes on the stem; the distance along the stem between nodes is known as an internode. Shoots develop from shoot meristem, which contains the apical shoot tip meristem from the epicotyl of the embryo, leaf primordia, and bud primordia, which are embryonic lateral shoot systems. Flowers, the reproductive organs of angiosperms, are modified shoots. Shoot meristems are located at the growing tips of stems, and in buds. Shoot Meristems The apical meristem is divided into two regions: tunica and corpus. The tunica meristem cells divided in a plane the produces additional surface meristem. The corpus meristem cells under the tunica divide in a plane that adds bulk to the shoot meristem. Generally there are two tunica layers and one corpus layer. As a shoot grows, buds are laid down by shoot meristem in the axils of leaf primordia. The repeating units of leaf and bud primordia are called phytomeres. These buds are dormant meristems that 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 Coleus stem tip, l.s. Shoot tip, xs SEM of shoot tip
The Shoot System: Primary Stem Structure - 2 The arrangement of derivative meristems is different in the stem from the root meristem zone. In the shoot meristem, procambium forms a cylinder of cells with ground meristem to the interior, and to the exterior of the procambium cylinder. As expected, the protoderm is the outer layer of derivative meristem. Single central strands of procambium extend out into the leaf primordia from the shoot meristem. 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. Growth regulators in the shoot tip and leaf primordia are responsible for the inhibition. Stem Functions The stem is the axis of the shoot system that provides mechanical support for and serves as the attachment site for leaves and reproductive shoots. Stems elevate 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 conduct solutes from leaves to storage and use sites. Stems are responsible for the overall growth (height and girth) of the plant from the primary and secondary shoot meristems There is much 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 generally primary growth structures.) Our discussion of stems will include both internal anatomy and external features of primary and secondary growth in stems. Primary Growth of Stems There are three basic primary growth patterns in stems: Early growth of most herbaceous dicot stems and some woody dicots Early growth of some woody dicots Monocot stems Most woody dicots Herbaceous dicots Monocots
The Shoot System: Primary Stem Structure - 3 While the arrangement of tissues in stems differs from roots, the tissues are the same, and by now, should be becoming familiar. Typical Dicot Stem Primary Growth Patterns Epidermis Cuticle for protection Some stomata for gas exchange Trichomes are common glands, prickles and hairs Cortex region Few layers of collenchyma for flexible strength Parenchyma layers Some sclerenchyma may be found in cortex, too Pith The pith is comprised of parenchyma tissue with many intercellular spaces. Some plants may have a hollow pith. An endodermis layer forms interior to the vascular tissue in these hollow-stemmed plants. Pith rays, extensions of pith between vascular bundles radiate from the pith to the cortex. Pith rays are difficult to observe in plants that have a cylinder of vascular tissue in primary growth. Helianthus stem, xs
The Shoot System: Primary Stem Structure - 4 Vascular Tissue Pattern 1: Ring of discrete vascular bundles with pith rays Vascular bundles may have a fiber bundle cap (which is called a phloem fiber cap because it is adjacent to the phloem) or a sheath of sclerenchyma surrounding the entire vascular bundle. Individual bundles are separated from each other by pith rays, parenchyma tissue that extends from the interior pith region of the stem to the cortex. Primary Phloem is found toward the exterior of the vascular bundle (rarely may have phloem on the inside of the xylem, too. Cucurbita stem vascular tissue is an example of this) Primary Xylem is found toward the interior of the vascular bundle 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 that lack secondary growth have closed vascular bundles, and no procambium remains between the primary xylem and primary phloem. Dicot open vascular bundle Closed vascular bundle
The Shoot System: Primary Stem Structure - 5 Pattern 2: Cylinder of Vascular Tissue Some dicots that have secondary growth, particularly woody plants, may have a complete cylinder of vascular tissue in primary growth separating the cortex from the pith. However, within the cylinder, whose interior and exterior borders are typically formed by a layer of sclerenchyma cells, are radiating rows of ground parenchyma that separate alternating xylem and phloem vascular bundles. It can be difficult to distinguish the bundles because there are no bundle sheaths or fiber caps. Primary stem in Tilia There is a reasonable amount of variation in secondary growth in dicots. Woody plants have extensive secondary growth. Most herbaceous dicots have open bundles and exhibit some secondary growth, although much of the secondary growth will be for support more than for conduction. The transition in stem growth pattern from primary to secondary is discussed with secondary growth.
The Shoot System: Primary Stem Structure - 6 Monocot Stem Variations Most monocots are reasonably small, herbaceous plants. Most monocots have no secondary growth, even perennial monocots. There are some notable exceptions, however, such as the palms. The common monocot families are the lily, grass and orchid families. Some distinctive monocot stem features: Monocot vascular bundles are closed. No procambium remains so generally monocots have 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. Vascular bundles have an anastomosing, or weaving pattern in longitudinal section. The parenchyma cells between vascular bundles are just referred to as ground tissue. This is the third vascular bundle pattern common in primary growth of stems. Most monocot vascular bundles contain two large xylem vessels toward the interior of the bundle, and some smaller vessels between them. The very first vessels formed usually collapse from stretching leaving an air space. As a result the vascular bundle often takes on the appearance of a "clown face". Phloem is always located toward the epidermis layer, and contains no fibers. A sclerenchyma bundle sheath surrounds each vascular bundle. Monocot stem, xs Monocot vascular bundle
The Shoot System: Primary Stem Structure - 7 Special Variations in some Monocots Hollow Stems A central pith cavity develops from cells of the ground tissue that are destroyed during early growth. This gives an appearance 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 nonapical growth and enlargement of cells throughout the plant stems. (Which is why we have to mow lawns). Thickened Meristems Some monocots achieve great dimensions without secondary growth. Although we will discuss this more when we discuss secondary growth and woody plants, one way in which a plant can have a larger girth is to produce a meristem that proliferates laterally just below the apical meristem. This thickened meristem is so large that the true apical meristem with its leaf primordia appears sunken into this area. The thickened meristem is procambium, which produces huge numbers of vascular bundles within the stem. This region of proliferating procambium is known as the meristematic cap.