Lecture 6 Video Part 1 Notes

Stem overview and functions

• The stem is the main above-ground axis in most plants and serves multiple roles across development and physiology.
• Primary roles:
  • Primary growth of the shoot and supports for leaves, flowers, and fruits.
  • Transport conduit for water, minerals, and organic compounds both within the stem and down to the roots.
  • Storage organ for carbohydrate reserves in some plants.
• Not all plants show an obvious stem; rosette-type plants have very short internodes but still possess a stem with leaves arising near the base.
  • In rosette forms, the distance between leaf attachment sites (internodes) is very small, giving the appearance of no stem.

Stem morphology: basic organization and terminology

• New growth begins from the terminal apical bud (apical meristem).
• As you move downward, you'll encounter regions where leaves and lateral buds (axillary buds) form.
• The stem is divided into nodes (regions of leaf attachment and axillary buds) and internodes (the stem segments between nodes).
• A building-block concept for stems is the phytomer:
  • A phytomer consists of the node, the internode below it, the axillary bud that forms in the axle, and the leaf.
  • The pattern repeats along the length of the stem, giving rise to modular growth (analogized to repeating floors in a building).
• Rosette-type plants still have stems; the distance between nodes/internodes is just very short.

Stem development: germination and the shoot apical meristem

• Stem development starts in the seed embryo and involves two apical meristems:
  • Root apical meristem at the base of the shoot-root axis gives rise to the radical/root system.
  • Shoot apical meristem at the opposite end gives rise to the shoot system.
• Lateral structures visible from the seed include cotyledons (seed leaves).
• The radical (primary root) emerges first from germination and is connected to the cotyledons by the hypocaudal.
  • Hypocaudal acts like a well or connector between root and shoot; it often assumes a bent structure when the seed emerges to protect the apical meristem inside the cotyledons.
  • After emergence, hypocaudal uncoils to allow the shoot system to form from the epicaudal (the region above the cotyledons).
• The shoot system forms from the epicaudal region above the cotyledons.
• Growth pattern in stems is modular, unlike roots which can form lateral roots more variably along their length.

Phytomers and the building-block concept of the stem

• Phytomer components (repeating module):
  • Node, internode, axillary bud, and leaf.
• The apical meristem initiates this sequence; over time, each phytomer matures into its components.
• Analogy: above-ground plant architecture resembles building floor plans repeating with each phytomer.

Internal anatomy: stages and tissue organization

• Stem development involves three longitudinal zones:
  • Zone of division (apical meristem region at the tip).
  • Zone of elongation (primary meristems form and primary tissues develop).
  • Zone of differentiation/maturation (primary tissues reach final development).
• Vascular arrangement in stems differs from roots:
  • Roots have a central stele with vascular tissue organized typically in a single strand.
  • Stems have discrete vascular bundles distributed throughout, to connect with leaves and buds that are positioned along the stem.
• In stems, vascular tissues are arranged in bundles rather than a single central strand, reflecting the need to supply multiple lateral structures.

Outer and ground tissues in stem anatomy (dicots)

• The outermost layer: epidermis (usually a single layer early in development; can thicken with additional layers later).
• Cortex: ground tissue located between the epidermis and the vascular tissue; contains parenchyma and sometimes collenchyma (the latter provides mechanical support, usually just beneath the epidermis and around stem corners).
• Vascular bundles: discrete units containing xylem and phloem.
• Pith: ground tissue at the center, typically parenchyma cells; some stems are hollow and lack a pith.
• In dicots (typical pattern):
  • Vascular bundles are arranged in a ring around the stem (a characteristic ring pattern).
  • Each bundle includes xylem on the inside and phloem on the outside.
  • The outer portion of each bundle may be associated with fibers (dark-staining sclerenchyma) for structural support.

Vascular bundle anatomy in dicots (sunflower as a dicot example)

• An individual vascular bundle shows the following arrangement (from outside to inside):
  • Epidermis
  • Cortex with thick-walled collenchyma near the edges (support tissues)
  • Vascular bundle consisting of:
  • Phloem tissue on the outside of the bundle (food-conducting tissue): includes sieve tube elements, companion cells, and often parenchyma.
  • A meristematic region called the procambium located between phloem and xylem, responsible for producing vascular tissues.
  • Xylem tissue on the inside of the bundle, with large, lignified cells (older metaxylem toward the center, younger protoxylem toward the outside of the xylem region).
  • Across the bundle, phloem fibers are often present on the outer side for additional support.
• The xylem and phloem arrangement (inside to outside) follows xylem inward and phloem outward within each bundle.

Monocot stem anatomy: corn stem as the classic example

• Cross-sections of monocot stems (e.g., corn) show a fundamentally different vascular organization:
  • No ring-like arrangement; vascular bundles are scattered throughout the cortex instead of forming a distinct ring.
  • Ground tissue is referred to simply as cortex; the boundary between cortex and pith is not clearly delineated as in dicots.
  • Each vascular bundle is surrounded by fibers, creating a “closed bundle” arrangement.
• Features of a monocot vascular bundle (in detail):
  • Phloem tissue on the outer side of the bundle with sieve tube elements and companion cells.
  • Xylem tissue on the inner side with large metaxylem elements and smaller protoxylem elements toward the outside of the xylem region.
  • The bundle is enclosed by sclerenchyma fibers (the “fibrous” sheath around the bundle).
  • A lacuna (air space) may be present within the bundle, giving the appearance of a tunnel-like structure.
• Closed bundles in monocots and the scattered arrangement are major reasons monocots generally do not form true wood or undergo classic secondary growth (though there are exceptions in some monocot trees like palms; they do not form typical wood as defined in woody dicots/gymnosperms).
• This structural layout contrasts with the dicot pattern and underpins differences in woody potential.

Monocots vs Dicots: quick structural differences (summary)

• Seed leaves (cotyledons):
  • Monocots: 1 cotyledon
  • Dicots: 2 cotyledons
• Root system:
  • Monocots: fibrous roots
  • Dicots: taproot system is common
• Stem vascular arrangement:
  • Monocots: scattered vascular bundles throughout the stem (no ring)
  • Dicots: vascular bundles arranged in a ring around the stem
• Floral parts (petals):
  • Monocots: petals in groups of 3
  • Dicots: petals in groups of 4 or 5
• Leaf venation:
  • Monocots: parallel venation
  • Dicots: net (reticulate) venation
• Woody potential:
  • Monocots: typically non-woody; true wood is rare or absent in monocots except for exceptions like some palms
  • Dicots: many can form true wood and undergo secondary growth

Visual and example references discussed in the lecture

• Sunflower (Helianthus) – a classical dicot stem used to illustrate ring-like vascular bundle arrangement.
• Corn (Zea mays) – a classic monocot stem with scattered vascular bundles and closed bundles surrounded by fibers.
• Alfalfa – dicot stem illustrating ring-like bundles (dicot pattern).
• Asparagus – monocot stem illustrating scattered bundles (monocot pattern).
• Fern rhizome – a primitive stem with a central vascular strand, highlighting an intermediate stem form between lower and higher plants.
• General reminder: a stem cross-section with central ring-like vascular bundles indicates a dicot; scattered bundles indicate a monocot; a central vascular strand in a rhizome-like structure can indicate a primitive stem form (fern).

Context: growth phases, development, and future topics

• Primary growth (occurs in both monocots and dicots) expands the length of the stem via the shoot apical meristem.
• Secondary growth (wood formation) is discussed in the second part of the lecture and relates to how cambium forms and produces growth rings in trees.
• The second part will cover modified stem structures and woody growth (secondary growth), including growth rings and differences between hardwoods and softwoods.

Quick study prompts and exam-style questions

• If shown an image of a stem cross-section, determine whether it represents a monocot or a dicot based on vascular bundle arrangement (ring-like vs scattered).
• Given a stem cross-section, identify the following structures: epidermis, cortex (parenchyma, collenchyma), vascular bundle, phloem, xylem, procambium, sclerenchyma fibers, and pith.
• Explain why monocots typically do not form true wood, using the concept of closed vascular bundles surrounded by fibers.
• Describe the sequence of stem development from seed to mature stem, including the roles of the apical meristem, hypocaudal, epicaudal, nodes, internodes, and phytomers.
• Compare monocot and dicot stems in terms of seed leaves, root system, vascular arrangement, floral parts, leaf venation, and woody potential, and give an example of each (e.g., sunflower vs corn, alfalfa vs asparagus).

End of Part 1: stem morphology and anatomy (lead-in to Part 2)

• The second part of the lecture will extend these concepts to modified stem structures and secondary growth, including the anatomy of twigs and branches, cambium formation, growth rings, and hardwoods vs softwoods.