Anatomy of Flowering Plants

The One Thing

Flowering plant structure is organised hierarchically from cells to tissues, tissue systems, and organs, with each structure adapted to its function. Differences in tissue composition and vascular-bundle arrangement explain the anatomy of roots, stems, and leaves in dicotyledonous and monocotyledonous plants.

Reactions, Processes and Experiments

Key Terms

• Plant anatomy: The study of the internal structure and organisation of plants.
• Meristematic tissue: Living tissue made of actively dividing cells responsible for plant growth.
• Apical meristem: Meristem at the tips of roots and shoots that causes increase in length, called primary growth.
• Intercalary meristem: Meristem between mature tissues, commonly at the bases of grass internodes or leaves, allowing regrowth.
• Lateral meristem: Meristem such as vascular cambium and cork cambium that increases the girth of stems and roots.
• Permanent tissue: Tissue formed from meristematic cells that has become specialised and generally loses the ability to divide.
• Simple permanent tissue: Tissue made of one main type of cell, including parenchyma, collenchyma, and sclerenchyma.
• Parenchyma: Living, usually thin-walled tissue involved in storage, photosynthesis, and repair; chlorenchyma contains chloroplasts and aerenchyma has large air spaces.
• Collenchyma: Living tissue with unevenly thickened cell corners that provides flexible support to young plant parts.
• Sclerenchyma: Usually dead tissue with thick, lignified walls that gives strength and rigidity; it includes fibres and sclereids.
• Complex permanent tissue: Tissue made of different cell types working together, mainly xylem and phloem.
• Xylem: Vascular tissue that mainly conducts water and minerals from roots to aerial parts and provides support; it includes tracheids, vessels, xylem fibres, and xylem parenchyma.
• Phloem: Vascular tissue that transports organic food from source regions to sink regions; it includes sieve tubes, companion cells, phloem parenchyma, and phloem fibres.
• Epidermal tissue system: The outer protective covering of the plant, consisting mainly of epidermis, stomata, and epidermal appendages such as trichomes and root hairs.
• Stoma: A pore in the epidermis controlled by guard cells that regulates gaseous exchange and transpiration.
• Ground tissue system: All tissues other than the epidermal and vascular systems, including cortex, endodermis, pericycle, medullary rays, and pith in suitable organs.
• Vascular tissue system: The conducting system formed by xylem and phloem, arranged as vascular bundles.
• Root anatomy: The internal organisation of a root, including epiblema, cortex, endodermis with Casparian strips, pericycle, radial vascular bundles, and a central pith.
• Stem anatomy: The internal organisation of a stem, including epidermis, cortex, vascular bundles, and pith.
• Leaf anatomy: The internal organisation of a leaf, including epidermis, mesophyll, and vascular bundles.
• Radial vascular bundle: Arrangement in which xylem and phloem occur on different radii, characteristic of roots.
• Conjoint vascular bundle: Arrangement in which xylem and phloem lie together on the same radius, characteristic of stems and leaves.
• Open vascular bundle: Vascular bundle containing cambium between xylem and phloem, allowing secondary growth.
• Closed vascular bundle: Vascular bundle lacking cambium between xylem and phloem, generally unable to show normal secondary growth.
• Endarch xylem: Xylem in which protoxylem lies towards the centre and metaxylem towards the outside, typical of stems.
• Exarch xylem: Xylem in which protoxylem lies towards the outside and metaxylem towards the centre, typical of roots.
• Secondary growth: Increase in the girth of stems and roots due mainly to vascular cambium and cork cambium.
• Vascular cambium: Lateral meristem that produces secondary xylem inward and secondary phloem outward.
• Cork cambium: Lateral meristem that produces cork or phellem outward and secondary cortex or phelloderm inward.
• Periderm: Protective secondary tissue consisting of phellem, phellogen, and phelloderm.
• Annual rings: Concentric layers of secondary xylem formed during successive growing seasons, often used to estimate the age of woody plants.
• Heartwood: Older, darker, non-conducting inner secondary xylem that mainly provides mechanical support.
• Sapwood: Younger, outer secondary xylem that conducts water and minerals.

Easily Confused

• Primary growth vs secondary growth: Primary growth increases length through apical meristems, whereas secondary growth increases girth through vascular cambium and cork cambium.
• Apical meristem vs lateral meristem: Apical meristem occurs at root and shoot tips, whereas lateral meristem increases the thickness of stems and roots.
• Intercalary meristem vs apical meristem: Intercalary meristem occurs between mature tissues and permits regrowth, whereas apical meristem is located at the tips and produces length growth.
• Simple permanent tissue vs complex permanent tissue: Simple permanent tissue contains one main cell type, whereas complex permanent tissue contains different cell types working together.
• Parenchyma vs collenchyma: Parenchyma is mainly associated with storage, photosynthesis, and repair, whereas collenchyma provides flexible support through unevenly thickened corners.
• Collenchyma vs sclerenchyma: Collenchyma is living and flexible, whereas sclerenchyma is usually dead, lignified, and rigid.
• Xylem vs phloem: Xylem mainly conducts water and minerals and provides support, whereas phloem transports organic food from source regions to sink regions.
• Radial vs conjoint vascular bundles: Radial bundles have xylem and phloem on different radii and are characteristic of roots; conjoint bundles have them on the same radius and occur in stems and leaves.
• Open vs closed vascular bundles: Open bundles contain cambium and can support secondary growth, whereas closed bundles lack cambium and generally cannot show normal secondary growth.
• Endarch vs exarch xylem: Endarch xylem has protoxylem towards the centre and is typical of stems; exarch xylem has protoxylem towards the outside and is typical of roots.
• Heartwood vs sapwood: Heartwood is older, darker, inner, and non-conducting; sapwood is younger, outer, and conducts water and minerals.
• Dorsiventral vs isobilateral leaves: Dorsiventral leaves have palisade and spongy mesophyll differentiated by surface, whereas isobilateral leaves have similar upper and lower surfaces and commonly parallel venation.
• Dicot vs monocot roots: Dicot roots commonly have diarch to tetrarch xylem, a small pith, and secondary growth; monocot roots usually have polyarch xylem, a large pith, and no secondary growth.
• Dicot vs monocot stems: Dicot stem bundles are commonly arranged in a ring and usually contain cambium; monocot bundles are scattered, usually closed, and surrounded by a sclerenchymatous sheath.
• Dicot vs monocot guard cells: Guard cells are generally kidney-shaped in dicots and dumbbell-shaped in many grasses and monocots.

What Gets Asked

• Identify a tissue from its structure and function: Marks depend on distinguishing thin-walled parenchyma, unevenly thickened collenchyma, and thick, lignified sclerenchyma rather than treating all supporting tissues as equivalent.
• Compare dicot and monocot roots: The required distinctions include diarch to tetrarch versus polyarch xylem, small versus large pith, and usually present versus generally absent secondary growth.
• Compare dicot and monocot stems: The key features are ringed versus scattered vascular bundles, the presence versus absence of cambium, and the sclerenchymatous sheath around monocot bundles.
• Interpret vascular-bundle terminology: Radial and conjoint describe the relative positions of xylem and phloem, while open and closed describe the presence or absence of cambium.
• Determine whether xylem is endarch or exarch: The mark-losing error is reversing the position of protoxylem and metaxylem; endarch is typical of stems and exarch of roots.
• Explain secondary growth: A complete answer must state that vascular cambium forms secondary xylem inward and secondary phloem outward, while cork cambium produces phellem outward and phelloderm inward.