BIOL 005B Lecture Notes - Lecture 22: Tracheid, Phloem, Parenchyma
BIOL 005B Lecture 22: Plant Structure
●Developmental plasticity
: the ability to alter form in response to environment
○E.g. Water lily leaves
○Developmental plasticity is more marked in plants than in animals
○In addition to plasticity, plant species have by natural selection accumulated
adaptations in morphology
■Water retention features: spines, water storage organs, shape
●Plant Organs
○The three basic plant organs: roots, stems, and leaves
○Morphology reflects their evolution as organisms that draw nutrients from below
and above ground
○They are organized into a root system and a shoot system
○Roots rely on sugar produced by photosynthesis in the shoot system, and shoots
rely on water and minerals absorbed by the root system
●Roots
○Roots are multicellular organs with important functions:
■Anchoring the plant
■Absorbing minerals and water
■Storing organic nutrients
○A taproot system consists of one main vertical root that gives rise to lateral roots,
or branch roots
○Adventitious roots arise from stems or leaves
○Seedless vascular plants and monocots have a fibrous root system characterized
by thin lateral roots with no main root
○In most plants, absorption of water and minerals occurs near the root hairs
■Vast numbers of tiny root hairs increase the surface area available for
water and nutrient uptake
○Root Modifications
■Prop roots that support tall, top-heavy plants (corn)
■Storage roots (carrot, beets)
■“Air” roots = pneumatophores (mangrove)
■Buttress roots (many rainforest trees)
■“Strangling” roots (some figs)
●Stems
○A stem is an organ consisting of
■An alternating system of nodes, the points at which leaves are attached
■Internodes, the stem segments between nodes
■An axillary bud has potential to form a lateral shoot, or branch
■An apical bud, or terminal bud, located near shoot tip and causes
elongation of young shoot
●Apical dominance helps maintain dormancy in most non-apical
buds
○Stem Modifications
■Rhizomes: horizontal stem just below the surface
