Stems

• plant stems evolved before leves and roots

• stems form architectural basis for shoots

  • all other plant organs attach to stems

• stems enable upqard and outward growth

  • apical meristems generate new tissues at stem and branch tips

    • apical growth (up) = better access to sun

Stem Anatomy

External

Vascular Tissues

• role of vascular tissues:

  • allow for internal movement of materials

  • provide mechanical support

• xlyem pulls from roots up to the leaves

  • transpiration stream

• phloem distributes sugar in solution from the source cells (in the leaves) to the sink cells (in the roots)

  • translocation stream

Xylem

• lignin is a complex phenolic compound

  • composition of monomers varies by species

  • adds strength and rigidity to cell walls

  • second most abundant organic material on earth

    • first = cellulose

  • removed in the manufactoring of paper

  • used as a binder for particlebpard and other composite wood products and as an adhesive for linoleum

  • used as a soil conditioner ans filler for phenolic resins

  • also used to make vanillin, a synthetic vanilla

• primary water-conducting cells:

• tracheids are elongated, tube-like cells with lignifed walls, tapered ends, and pits that allow water to move laterally between adjacent cells

• vessel elements (a.k.a “tracheae”) are shorter, wider, and arranged in continuous tubes that facilitate more efficient water conduction

• feature perforated end walls that allow for smoother water flow (reduced resistance)

• xylem fibers are long, thick-walled cells that provide structural support for withstanding external forces (e.g. wind or gravity)

• xylem parenchyma stores startches, fats, and other nutrients, aids in lateral conduction of water, and helps repair damaged tissues

• two stages:

  • protoxylem forms first

    • narrow elements wih annular/spiral thickenings of lignin

    • stretch and elongate as the organ grows

  • metaxylem forms later

    • contains wider vessesls/tracheids with reticulate or pitted thickenings

    • found in mature regions where elongation has ceased

• endarch xylem is typical of stems

  • protoxylem lies towards the center (pith) and metaxylem towards periphery

  • growth occurs outward from the center

• opposite in roots, where growth occurs inward

  • exarch xylem

• how does water move through the xylem?

• absorption in the roots creates a postitive pressure that pulls water upqard through the xylem

• adhesion and cohesion of water molecules within narrow vessels and tracheids allows water to rise

• evaporation of water from leaves (transpiration) pulls water upward through xylem as it creates a negative pressure

  • this is the most significant driving force for maintaining a continuous flow from roots to leaves

Phloem

• what makes up the phloem?

• sieve tube elements are elongated, tube-like cells arranged end-to-end to form continuous channels

  • perforated end walls (sieve plates) facilitated food transfer between adjacent sieve tube elements

  • lose nuclei as they mature (reduces cellular interference)

• companion cells provide metabolic support for te sieve tube elements (lots of mitochondria)

  • connect to sieve tube elements via plasmodesmata

• phloem fibers (a.k.a bast fibers) are dead components that are elongated, thick-walled cells providig mechanical strength to help withstad external forces

  • forms many commericall products

    • e.g. jute, flax, and hemp

• phloem parenchyma functions in storage of starches, lipids, and proteins and in lateral movement of organic solutes

  • contributes to healing/tissue regeneration in some plants

  • absent in most monocots

• protophloem forms first

  • narrow elements wih thin walls found in active growth regions

  • often get crushed as an organ matures

• metaphloem forms later (after elongation stops)

  • contains wider sieve tubes and companion cells

  • remains functional in actively growing organs

• movement is bidirectional

  • nutrients move form leaves to roots, flowers, or fruits during growing seasons

  • nturients can be transported back from roots during dormant seasons

• transport of solutes requires ATP

• pressure flow hypothesis

  • step 1: sucrose is loaded into sieve tube elements, which increases solute concentration causing waterr to enter the phloem from nearby xylem vessels

  • step 2: pressure gradient is created, forcing sugar solution to move through sieve tubes

  • step 3: sucrose is unloaded into sinks (e.g. roots, fruits, etc.) throguh active transport; solute concentration in sieve tubes is now lower, causing water to exit phloem back into xylem

  • step 4: water is recirculated as the water pressure at the sinks drop and water re-enters xylem

• symplastic/symplasmic loading

  • sucrose loaded from the cekks that produce them directly into sieve elements or indirectly via companion cells via plasmodesmata

  • no crossing of cell membranes or cell walls

    • no ATP needed

  • common in woody plants

• apoplastic/apoplasmic loading

  • sucrose secreted by parenchyma cells (SWEETs) loaded from intercellular spaces into phloem

    • SWEETs - Sugars Will Eventually be Exported Transporters

  • ATP used for active transport into companion cells

  • needed for high sucrose concentrations

• how does sucrose get into phloem?

• polymer trapping

  • smaller sugars move symplastically into companion cells and are converted into larger oligosaccharides and get trapped after going through the plasmodesmata

  • specialized type of symplastic loading

Vascular Bundles

• functional units of transport in plants

• highly variable morphology

• classified based on presence/absence of cambium

  • cambium is a meristematic tissue that allows for secondary growth → increases girth

  • open vascular bundles contain cambium between the xylem and phloem

    • typical in diotyledonous stems

  • closed vascular bundles lack cambium, meaning they cannot undergo secondary growth (rely more on elongation)

    • typical in monocotyledonous stems

• also classified based on organization of xylem and phloem

  • concentric vascular bundles arranges tissues in rings, one completely surrounding the other

    • less common (mostly in monocot stems and some ferns)

    • closed type of vascular bundles and no secondary growth

    • amphivasal (leptocentric) - phloem surrounds xylem

    • amphicribral (hadrocentric) - xylem surounds phloem

• also classified based on organization of xylem and phloem

  • radial vascular bundles arrange tissues in separate, alternating strands that radiate out from the center

    • typically found in roots

  • conjoint vascular bundles arrange tissues side by side, usally along the same radius

    • common in stems and leaves

    • further distinctions based on the presence of a cambium and potential for secondary growth

• conjoint vascular bundles may be:

  • collateral bundles (most common) have phloem only on the outer side of the xylem

    • may be open or closed ased on presence/absence of camium

  • bicollateral bundles have phloem on both sides of xylem

    • cambium on both sies (inner and outer)

    • allows for most effiecient distribution of nutrients, which is particularly good for plants wih large, wide leaves

    • found in few families, such as cucurbinaceae (pumpkins, cucumbers, and other gourds)

Wood and Bark

• secondary meristems involved in seondary growth

  • vascular cambium - produces secondary xylem (wood)

  • cork cambium - produces secondary phloem (inner bark)

Vascular Cambium

• fusiform initials - cells that generate secondary xylem (secondary tracheids and vessel elements) and secondary phloem (sieve tubes)

• ray initials - cells the generate vascular rays, which store starches, proteins, and lipids and also transport food, water,a nd minerals lateraly between and secondary xylem and secondary phloem

• sapwood - actively conducting xylem

• heartwood - older xylem clogged by tyloses generated by neighboring parenchyma cells (can no longer conduct water)

  • impregnated with decay-resistant chemical compounds)

Cork Cambium

• some parenchyma cells produced tp the inside

• cork - produced to the outside of the cork cambium

  • cork cells have cell walls made of lignin and suberin

    • suberin = phenolic compounds + waxes

  • tannins aslo found in cork tissues

  • exude gums and latexxes

  • interruptions in this layer that provide gas exchange for inner stem tissues are called lenticels

• parenchyma cells + cork cambium + cork = periderm

  • most woody plants produce a series of periderms over time

    • successive layers of dead periderm form the outer bark

• non-woody, but tall plants

  • some plants sacrifice branching to reduce drag from external forces and instead produce thickening tissues before the stems increase in height and ovrelap leaf bases to provide protection and support

  • tree ferns, cycads, and palms lack a vascular cambium

Modified Stems

• rhizomes - underground stems that grow horizontally

  • function in storing nutrients and water

  • e.g. ginger and turmeric

• stolons - horizontal stems that grow above ground or just below the surface of the soil and are used for asexual reproduction

  • e.g. some grasses

• tubers - rounded/cylindrical structures on the tips of stolons that eventually detach from the main structure

  • store starches for surviving drought/cold

    • e.g. potatoes

      • “eyes” = nodes and axillary buds

• bulbs - short stems with many fleshy leaves

  • may feature a basal plate (short, flattened stem near the root)

• corms - underground stems that grow vertically

  • store nutrients for survivng drought and cold

  • e.g. taro and water chestnut

• cladodes - flattened stems that perform photosynthesis and may feature spines

  • e.g. cactus pads