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Ch. 24 Notes: Plant Structure & Function
Characteristics of Plants

Multicellular, Eukaryotic, and Photosynthetic autotrophs
Share many characteristics with their green algae ancestors:

Have chloroplasts with chl.a, chl. b, and carotenoids

Cell walls containing cellulose

Store food as starch in plastids
Plants have evolved complex bodies with cell specialization for different functions. New
adaptations/structures were needed to help with the move to land:
1) Transitionfrom Aquatic Ancestors to Land by Bryophytes:
Nonvascular, small (mosses, hornworts, and liverworts)  475 mya

NO true roots, leaves, or stems

A new mode of reproduction was necessary: Embryophyte = embryo is
retained inside female’s ovule for protection and to prevent dessication

Alternation of Generations = a haploid gametophyte generation
produces and alternates with a diploid sporophyte generation. The
sporophyte, in turn, produces gametophytes.
2) Seedless Vascular Plants: Ferns and Horsetails  400 mya

Regional specializations of the body developed to extract the resources needed
for photosynthesis from the terrestrial environment.
o Roots, Shoots, and Leaves (division of labor)

Vascular Tissue = special tissues for the (long-distance) transport of water and
nutrients through the plant body (between leaves and roots)

Lignin = added to cell walls of “woody” plants to help them stand up in the
terrestrial environment
3) Origin of the Seed: Gymnosperms  360 mya

Seed = plant embryo packaged with a store of food within a resistant coat

Gymnosperms = seeds as naked structures
o Include the Conifers, Cycads, Ginkgoes, and Gnetophytes
4) Emergence of Flowering Plants: Angiosperms  130 mya

Flowers evolved  attract pollinators for better fitness

Unlike gymnosperms, they bear seeds within the flower’s protective
ovules (ovaries)
ANGIOSPERM BODY PLAN

Each system depends on the others for survival of the whole plant
Roots

Anchors plants

Absorbs and conduct water and nutrients

Stores food

Produce hormones to regulate growth

Absorption of water is greatly enhanced by root hairs = projections of epidermal
cells
o Increase the surface area of the root.
o Ex. Rye plants  14 billion root hair cells = 10,626 km!

Water and mineral absorption are also enhanced by mycorrhizae = symbiotic
associations between roots and fungi

Root nodules = symbiotic associations of plant roots and bacteria capable of
fixing nitrogen
Stems

Axillary bud = An embryonic side shoot found in the angle formed by each
leaf and the stem; usually dormant
o When triggered, can produce a new branch
Leaves

Main site of photosynthetis
Tissue Systems

Continuous throughout the plant
1) Epidermal Tissue System

Single layer of tightly packed cells covering and protecting the young parts of
the plant
o Includes Root Hairs
o Aerial parts covered by the waxy cuticle
o Trichomes = protect plant from too much sun and may help protect
plant from herbivores by making a toxic substance

Found on stems, leaves, and reproductive organs

Look like “hairs” or small thorns

Density of these increases on plants as a function of herbivory
2) Ground Tissue System

Occupies the space between epidermal and vascular tissue systems

Function in photosynthesis, storage, and support
3) Vascular Tissue System

Xylem = transports water and mineral from roots to leaves
o Made of Tracheids and Vessel Elements

Both are dead at functional maturity
o Also function in support

Phloem = transports sugar and other organic compounds
o Sieve-Tube Members = Chains of phloem cells

Cells are alive at functional maturity.
o Lack a nucleus, ribosomes, and a distinct vacuole.

At least one Companion Cell is connected to each sieve-tube member
by many plasmodesmata
o The companion cell’s nucleus and ribosomes also serve the
sieve-tubes member, which lacks these organelles.
o Helps load sugar produced in the mesophyll into sieve-tubes of
leaves of some plants.
PLANT GROWTH
Begins with germination of the seed and continues for the lifespan of the plant.

Indeterminate growth is made possible by meristems = perpetually embryonic
tissues

Primary Growth = elongation; increase in height/length

Secondary Growth = increase in girth (circumference)
Tissue Organization of Leaves

Cloaked by a waxy cuticle
o Protects against physical damage, pathogens, and water loss

Stomata are pores flanked by guard cells which regulate gas exchange with the
surrounding air and photosynthetic cells inside the leaf
o Allow for transpiration (water loss from the plant by evaporation)
o More on the bottom surface of leaf to minimize water loss

Vein = vascular bundles with xylem and phloem
o May be wrapped in bundle sheath cells in C4 plants