Download primary structure 16 March

Shoot Primary Structure and Development Shoot Primary Structure and Development apical meristems Phytomeres: repea5ng modular units of plant structure. consist of a node, the leaf a;ached at that node, the internode below that leaf, the axillary bud at the base of the internode. Stem apical meristem produces phytomeres as well as reproduc5ve structures. Bud = dormant apical meristem protected by mul5ple layers of specialized leaf primordia (bud scales) Shoot Primary Structure and Development Shoot Primary Structure and Development An.clinal divisions: Produce more cells laterally within a layer. Tunica layers – divide an5clinally Periclinal divisions: Pruduce addi5onal layers of cells. Corpus – cells divide periclinally, adding cells to the corpus * * Shoot Primary Structure and Development Shoot Primary Structure and Development Older internodes are longer due to cell elonga5on. Many complete phytomeres may be present within an apical bud or axillary bud. When the buds “break,” the cell of the organs within expand and complete their growth and differen5a5on. Internode elonga5on and leaf expansion can be occurring simultaneously in mul5ple phytomeres. Shoot Primary Structure and Development leaf gap Shoot Primary Structure and Development Tissue organization in primary stems
Shoot Primary Structure and Development Tilia primary stem 5ssues. Medicago primary 5ssues. Leaf vena5on Arrangement of vascular 5ssues in leaf veins Leaf Tissues Epidermis: guard cells (stomata) and reg. epidermal cells. OPen a thick cu5cle. Mesophyll: spongy and palisade layers of photosynthe5c parenchyma. Veins: note arrangement of xylem and phloem rela5ve to stem vascular bundles. Other 5ssues: “mul5ple epidermis,” bundle sheath extensions, sclereids, trichomes, etc. provide structural support, some5mes water storage, inhibit water loss, etc. -­‐ trichomes inhibit water loss by crea5ng a barrier of s5ll air -­‐ Stomatal crypts: sunken areas of the leaf, another sheltered space Leaf Tissues Shoot Primary Structure and Development Bulliform cells in grasses -­‐ lose water (and turgor pressure) when plant experiences water stress. -­‐thin cell walls allow them to contract instead of undergoing plasmolysis. -­‐Folds the leaf, sheltering the stomata and inhibi5ng further water loss. Leaves can be adapted for different condi5ons Sun vs. shade Sun: mul5ple palisade layers Shade: thinner leaves, less investment in chlorenchyma 5ssues Phyllotaxy Phyllotaxy: orderly pa;ern of leaves arranged on a stem. Phyllotaxy is determined in the L1 layer of the apical meristem. -­‐ ini5a5on of leaf primordia influenced by hormones (auxins) and cell wall modifica5ons (expansins; disrupt H-­‐bonds between cell wall polysaccharides). Alternate: one leaf per node; angle of leaf at successive nodes changes (spiral, helical). Opposite: two leaves per node, opposite sides of stem. Whorled: three or more leaves per node. Simple leaves Shoot Primary Structure and Development Pe5olate: leaves with a pe5ole Sessile: leaves lacking a pe5ole Compound leaves Compound leaves Bud scales: modified leaves Aesculus (chestnut)
The young shoots are tightly
packed in the buds and are
protected by bud scales,
which are highly modified
leaves initiated late in the
previous growing season.
The buds open to reveal
the oldest rudimentary
leaves, in response to day
length or warming in spring.
Internodal elongation has
separated the nodes from
one another. T
Defensive stems, leaves, and epidermis Modified leaves and stems Tubers: underground stems modified for storage of starch Tuberous roots: roots with expanded storage 5ssues. Rhizomes: horizontal stems at or below the ground surface (for “movement,” veg. propaga5on) Stolons: horizontal aboveground stems (strawberries, spider plant, for veg. propaga5on) Bulb: modified shoot with short internodes and fleshy storage leaves (onion), for overwintering Corm: modified underground stem (ver5cal but short, expanded storage 5ssue) More modifica5ons Tendrils: modified stems or leaves Cactus spines: modified leaves More modifica5ons