Topic 6 - The histology of plant organs (regarding to nutrition value and digestibility) Flashcards
Histology of the root
Root tip: Initial stem cells → meristematic tissues → permanent tissues
Protection: root cap
The root cap
A parenchymal tissue with amyloplasts which sediment to the lower side of the cell
Accumulation of Ca++ → auxin release → gravitropism
Mature region of the root cap
Dermal tissue (rhizodermis)
Cortex
Stele
Dermal tissue (root cap)
Generally one cell layer
Asymmetrci divisions → root hairs (extensions of dermal cells) → increase absorption surface area
Root hairs live only a few days
Cortex
Multiple layers of parenchymal cells (stored nutrients, starch, crystals, latex) or aerenchyma (for water plants)
Transport water and minerals to the vessels
Stele
Outer most layer is the pericycle → produce branch roots and secondary dermal tissue
Pith: parenchymal cell layers for storage
Simple xylem and phloem bundels alternate with each other
older root
Cellular division in the pericycle → new root cap and root tissues
Generally decreasing digestibility (higher proportion of cell with thickened walls)
Pericycle activity
Production of vascular cambium → secondary transport vessels → thickening of the root
Tissues of storage roots
Generally lack of mechanical tissues (e.g.: carrot)
Sugar beet → cambia are arranged in concentric circles
More parenchymal cells → better digestibility
Histology of the stem
Shoot apex: initial cells (tunica, corpus) → meristems → permanent tissues
No protective cell layer
Epidermis (stem)
Generally one cell layer
Similar to the leaf, but less stomata
Cuticle, wax, appendages
Cortex (stem)
Monocots - not differentiated into cortex and pith → storage parenchyma
Dicots - below dermal tissue: chlorenchyma, mechanical tissues and less storage parenchyma
For water plants: aerenchyma
Stele (stem)
Pith (parenchymal) + vascular bundles with rays
If hollow → pith cavity
Secondary growth (stem)
Rare in monocots
Four major types in dicot plants
Secondary growth → higher proportion of lignified cells → decreasing digestibility
Differentiation of the dermal tissues (stem)
Epidermis → periderm → rhytidome
Rupture, drop off
Tannins, resin → used as a spice or as medicine
Histology of the wood
Soft wood: less fibers, less lignification
Hardwood: more fibers, strong lignification
Heartwood: no water conduction parenchyma cells block the vessels
Sapwood: conduction of water and nutrients
Histology of leaves
Determined growth - meristematic activity stops
Young leaves
Primordia
Dermal tissue (leaves)
Mainly one cell layer
Cuticle, wax, appendages
Hypostomatic (underneath)
Epistomatic (upper surface)
Amphistomatic (both)
Nervation (leaves)
Transport, mechanical stability
Closed vascular bundle + bundle sheath
Mesophyll (leaves)
Middle leaf
Homogenous
Heterogeneous
Assimilation, gas exchange
Secretory cavities, crystals
Kranz (leaves)
Leaf anatomy
In C4 plants: bundles surrounded by two rings of cell with different plastds
Better CO2 fixation
Species are tolerant to drought
Tissues of the flower
Sepals, petals, tepals Stamens, pollen Pistil Fruits Seeds
Sepals, petals, tepals
Similar structure to leaves
One layered epidermis with papillae and hairs
Pigments in vacuoles → e.g. carotenoids → nutrient supply
Lack of mechanical tissues
Altogether poor nutritive value
Stamens, pollen
Stamens are modified leaves
Pollen wall → outer layer consists of sporopollenin
Chemically very stable
Water-solulbe proteins → allergy
Sporopollenin
Biosynthesised of fatty acids, phenolics and carotenoids
Pollenkitt
Adhesive material around the pollen
Pistil
A modified leaf
Stigma
Tissue of the ovary → fruit wall
Chlorenchyma → storage parenchyma
Stigma
An epidermis with cuticle and glandular hairs → sticky surface to catch and trap pollens
Fruits
Three regions
Exocarp
Mesocarp
Endocarp
Secretory cavities
Exocarp
One cell layer covered with cuties, wax, hairs, or it can be lignified
Mesocarp
In fleshy fruits it’s a multilayered parenchyma storing pigments, and nutrients (fatty acids, CH, proteins)
In dried fruits it consists of sclereids
Can be fused to endocarp
Endocarp
One or multilayered cells
Can be fused to mesocarp
Hard, stoney endocarp consists of sclereids
Taxa (and fruit) specific tissues
Fleshy fruits (e.g. berry): storage parenchyma
Dried fruits (e.g. capsule): sclerenchyma
For some types (e.g. drupe) the two types are combined
Fibers (fruit)
Contraction helps fruits to split open (e.g. pods)
Seeds
Seed coat + (perisperm (2n)) + endosperm (3c) + embryo
Multilayered storage parenchymal cells
Taxon specific cells (seeds)
Seed coat can be covered with cuticle, wax, mucilage
Parenchyma + sclereids (seeds)
One layer, multilayered dry or fleshy
Pigmented cells with tannins
Thick seed coat → digestive problems (shelled seeds)
Taxonspecific composition (seed)
Carbohydrates, proteins, fatty acids → seed used in animal nutrition (cereals, pulse crops)
Absent endosperm and perisperm
Nutrients stored in the embryo (cotyledons)
Thin hemicellulose walls
Good digestibility
Metabolised rapidly → release of nutrients
