Plants: Plants: Evolution, Classification, Evolution, Classification, Anatomy & PhysiologyAnatomy & Physiology

Biology SAT Lecture, 2006Biology SAT Lecture, 2006

Plants Overview

• Evolution of plants and plant classification

• Plant anatomy: – Tissues, Roots, Stems, Leaves

• Transport in plants

• Plant growth

What is a plant?• Plants are multicellular, eukaryotic

photoautotrophs• Plants have a waxy cuticle covering that helps

them retain water• Gas exchange occurs through holes, or stomata,

in the leaf surfaces• They have organs such as roots, stems and leaves• A vascular system carries water and minerals up

and nutrients down (and sometimes up again!)• Reproductive structures, called gametangia,

contain gamete-producing cells

What is a plant?

Plant Evolution: Appearance of the major plant groups

Byrophytes Byrophytes (non-tracheophytes)(non-tracheophytes)

MossesMosses

Vascular Plants Vascular Plants (tracheophytes)(tracheophytes)

Seedless Seedless Vascular PlantsVascular Plants

Ferns, etcFerns, etc

Plant KingdomPlant Kingdom

Gymnosperms Gymnosperms (“naked seed”)(“naked seed”)

Conifers, etcConifers, etc

Angiosperms Angiosperms (“vessel seed”)(“vessel seed”)

Flowering plantsFlowering plants

Monocots Monocots (“one cotyledon”)(“one cotyledon”)

Grasses, orchids, bamboo, palms, Grasses, orchids, bamboo, palms, lilies, grainslilies, grains

Dicots Dicots (“two cotyledons”)(“two cotyledons”)

Most shrubs & trees, many food cropsMost shrubs & trees, many food crops

Angiosperms are broken into 2 groups: monocots and dicots

Monocots & Dicots

• Cotyledons are the embryonic seed leaves• Monocots: orchids, bamboos, palms, and

lilies AND the grasses, including wheat, corn and rice (these are WAY important, for obvious reasons!)

• Dicots: all the rest…(roses, cabbage, beans,

potatoes, most fruit trees, etc) So if you’re asked which is more common, choose this one

Seed germination begins the life of a new plant

The plant: root system & shoot system

Modifications of roots and stems• Many roots are adapted for storing food

• Rhizomes are actually modified stems that store food

• Modified stems can also act as “runners”, allowing a plant to reproduce asexually

Parenchyma = jack of all trades (storage, cell resp, photosyn,...), most abundant cell typeCollenchyma = support plant in growing partsSclerenchyma = strengthens & supports in mature wood; ex) fiber & sclereid cellsWater-conduction cells (we’ll see these in a moment, another slide)

Main Plant Cell Types

Three tissue systems make up a plant:

Epidermal tissue = outer layer of r, s, l

Vascular tissue = conducts water & food (xylem & phloem)

Ground tissue = the bulk of a young plant, fills in space between epidermis and vascular tissue

How do things move around inside plants?

• XYLEM tissue = Water-conducting: made of tracheids and vessel elements – are dead, hollow tubes when mature

• PHLOEM tissue = Food-conducting: made of sieve-tube members and companion cells – are alive when mature

• Ferns, gymnosperms and angiosperms have these vascular tissues and are called tracheophytes.

• Bryophytes (mosses) do not have xylem and phloem and so they are non-tracheophytes

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Roots anchor the plant and allow for absorption of water & nutrients from soil

Dissolved nutrients move into root by active transport … water follows by osmosis! Beautiful.

From root hairs into xylem – then up!! The endodermis acts as the gate-keeper

Tissues of the leaf

• Guard cells regulate opening and closing of stomata (openings for gas exchange: O2, CO2, H2O vapor)

• Mesophyll cells, the ground tissue of leaves, contain chloroplasts and perform photosynthesis

Guard Cell Action: Bringing K+ into guard cells causes water to follow by osmosis; cells become turgid and stoma opens. Magic! (Flaccid guard cells = closed stoma … not a bad idea if you’re not photosynthesizing and trying to conserve H2O ... Like at night!)

How does water get to the top of trees? Combo of root pressure, capillary action & transpiration pull!

Plant growth is “indeterminate” growth - never ceasing (at root & shoot tips at least)

• Plants grow in length (primary growth) and in thickness (secondary growth)

• Meristem tissue: unspecialized cells that divide to generate all kinds of new plant tissue– Apical meristem: meristem at the tips of roots and

shoots, allow them to grow in length– Vascular cambium & Cork cambium: meristem

that allows stems & roots to thicken and branch out

Roots & stems exhibit both primary and secondary growth

Root Cross-section

Stem Cross-section

Vascular cambium makes xylem & phloem while cork cambium makes… cork!

Why do trees in temperate climates have annual rings?

Plant hormones cause growth

• Auxins: directs stem & root growth, responsible for phototropism (growth towards light) and geotropism (growth up away from gravity).

• Gibberellins: stimulate stem elongation, help end dormancy of seeds & buds; found in apical meristems of buds, roots, and leaves and in plant embryos.

• Cytokinins: promote cell division & leaf expansion; determine timing of cellular differentiation

• Ethylene: stimulates the ripening of fruit and the abscission (dropping) of leaves, flowers, and fruits.

Plant Tropisms

Can you guess what these are?• Phototropism:

• Gravitropism:

• Thigmotropism:

Plant Growth vocabulary

•Annuals: complete their life cycle in one year•Biennials: complete their life cycle in two years, usually flowering during the 2nd year•Perennials: plants that live many years