Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance (connect root and leaf vasculature) 3. Storage (some species) 4. Photosynthesis (some species) But lots of plants don’t have any stems at all – if not required, why “waste” the resources? While all the above functions are important, the most general benefit is improved light environment for the leaves – especially due to competition between plants.
Shoot growth – primary plant body Shoot – above ground plant structure stems, branches, leaves Apical Meristems make primary shoot increase in length Lateral Meristems make secondary growth increase in girth 35.11/35.10
Apical Meristem makes primary shoot 1. stem growth and tissues 2. leaf primordia – become leaves 3. bud primordia – become lateral branches 35.16/35.15
This process creates the primary plant body with it’s node – internode structure Node – leaf/branch attachment Internode – between nodes 35.2
Apical dominance Apical meristem at the leading shoot tip inhibits bud primordia (lateral buds) nearer to the tip, releasing them later (farther) resulting in “Christmas tree” like growth form. 39.9
35.17/35.16 Stem sections show all three main tissue types vasculature - often in bundles or sometimes rings epidermis ground tissue – often called cortex, or “pith” if inside
These sections are useful, but mask the 3-dimensional structure The vascular bundles appear separate in cross section, but generally have multiple cross connections throughout stem, including in monocots, creating an elaborate meshwork, including connections to the leaves.
Modified Stems Tendrils and twining stems Thorns (vs. spines) Stolons – above ground runners Rhizomes – below surface runners Food storage Tubers – swellings of stolons & rhizomes Corms – swellings at base of stem
Water storage (succulence) Cacti – stem modified for water storage and photosynthesis (leaves are spines). Two other families, Spurge (Euphorbiaceae) and milkweeds (Asclepiadaceae) have also evolved this. Classic example of convergent evolution.
SECONDARY GROWTH Secondary growth – increases in girth (width) of stems and roots resulting from lateral meristems [aside] Simple non-rigorous categories, widely used 1.Woody plants – with 2o growth – trees and shrubs 2. Herbaceous plants – little or no 2o growth, “herbs” a. Grasses b. Forbs (herbaceous dicots)
Two main Lateral Meristems control secondary growth 1. Vascular cambium – makes 2o xylem and phloem 2. Cork cambium – makes periderm In the secondary plant body, these form concentric cylinders or sheaths (rings in cross section) of meristem 35.11/35.10
Vascular Cambium Fusiform initials – key meristematic cells, vertically elongated. They produce xylem cells to the inside and phloem outside, causing increases in girth. Vascular cambium phloem (cross-section) xylem 35.20/35.19
Wood – secondary xylem Xylem – remains in place and continues to function for many years. Rigid cells build on one another expanding the stem. 35.22/35.20 Eventually the tree may stop using the inner xylem. 1. Sapwood – outer conducting xylem 2. Heartwood – inner older xylem, no longer conducting. Often darker due to deposits into vessels to block conductance and pathogen transmission
Growth rings In seasonal climates (cold/warm, wet/dry), early season growth, “early wood”, is less dense (larger cells, thinner walls) than “late wood”, producing annual “rings” visible to the eye. Used to create detailed climate records. Bristlecone pine can live 4900 years. Used to reconstruct climate to 8000 years ago.
Fig 35.21 – a composite ring-width chronology for Mongolian conifers suggesting elevated temperatures in 1900s.
Phloem – usually only the current year phloem functions in sugar transport. It is eventually pushed outward and collapses due to the expanding stem. 35.19/35.18
Cork Cambium – the Periderm A second sheath of meristem develops in the secondary phloem, outside the vascular cambium, called the cork cambium. It produces the periderm, which replaces the epidermis in secondary growth. Cork cells – main component of the periderm, produced by the cork cambium to the outside. They are lined with suberin and dead at maturity. Impermeable to water (and gases). Lenticels – openings in the cork layer to allow the living cells of the stem to respire
Bark – all tissue outside the vascular cambium -what can be removed without killing the tree As the tree expands, this tissue is stretched and eventually split and sloughed off. Structure of periderm determines the pattern.
Cork cambium This figure nicely summarizes the structure of a woody stem