Download Shoots – Chapter 25

Shoots – Chapter 25
Shoot
 Includes stem and leaves
 Two principal functions of stems are support and conduction
 The principle function of the leaf is photosynthesis
Primary structure of stem
 Lots of variation in structure
– Vascular system continuous cylinder
– Vascular tissues a cylinder of bundles
– Vascular tissues bundles scattered throughout ground tissue
Continuous vascular cylinder
 Vascular bundles actually separated by narrow parenchyma
– These areas are termed interfasicular regions
 Primary xylem forms from inner procambium
 Primary phloem forms from outer procambium
Circular bundle arrangement
 Pith rays (interfasicular regions) are wide
 Vascular cambium partly procambial and partly interfascular in origin
Scattered bundle arrangement
 Vascular bundles closed
– Do not give rise to a cambium
Leaf arrangement
 Arrangement of leaves on stem is called phyllotaxy
– Helical
– Distichous
 Two opposite ranks
– Opposite
– Decussate
 At right angles
– Whorled
Leaf structure
 Leaf consists of blade and petiole in eudicots
 Sessile leaves lack petioles
 Stipules can form at the base of the leaf
Leaf structure
 In monocots, base of leaf is expanded into a sheath
Simple leaves
 Blades are not divided into distinct parts
 Can be deeply lobed
Compound leaves
 Pinnately compound
– Leaves arise from rachis
 Palmately compound
Compound leaves
 Buds are formed in axils of leaves and not of leaflets
 Leaflets all arise from same plane, while leaves arise from stem in various
planes
Leaf epidermis
 Compactly arranged and covered with cuticle (as are stems)
 Stomata are openings that facilitate gas exchange
– Can occur on both sides of leaf or only one side
– If on one side, usually on lower side
– Trichomes help retard water loss
Stomata arrangement
Aquatic plant
Xerophyte
Mesophyll
 A ground tissue
 Site of photosynthesis
– Lots of extracellular space
– Lots of chloroplasts
 In eudicots, mesopyll is differentiated into palisade parenchyma and spongy
parenchyma
– Most photosynthesis occurs in palisade parenchyma
Xerophyte
Grasses
Vascular bundles
 Veins are scattered throughout leaves
– Netted or reticulate venation in eudicots
– Parallel or striate venation in monocots
 Minor vs. major veins
Veins
 Usually xylem on top, phloem on bottom
 Veins not exposed to intercellular spaces of mesophyll
 Surrounded by bundle sheath
Bundle sheath extensions
 Mechanical support
 Conduct water to epidermis
C4 vs. C3 leaves
 Mesopyll cells and bundle sheath cells form two concentric layers around
vascular bundles (Kranz anatomy)
 Lots of big chloroplasts in BSC
 Veins close together (no more than 4 mesophyll cells apart)
 Mesophyll and bundle sheath cells not concentracally arranged


Small chloroplasts in BSC
Veins farther apart
C4 vs. C3 plants
 C4 plants export photosynthates (products of photosynthesis) faster
 C4 plants have bulliform cells
Environmental factors
 Phenotypic plasticity
Light
Abcission
 Leaf scar remains
Transition region
Flower
 Development of the flower ends the meristematic activity of the shoot apex
Stem and leaf modifications
 Tendrils
– Arise from stem or leaf
Cladophylls
 Modified stems
 Perform photosynthesis
Spine
Thorn
Tuber
 Tuber inside periderm is storage parenchyma
 Eyes contain group of buds
 Scale like leaf
Bulb
 Thickened leaf tissue
Corm
 Thickened stem tissue
Thickened petioles
Water storage