PLANTS basic Evolution & Classification Anatomy & Physiology of Roots, Stems & Leaves

Plants Overview Evolution of plants and plant classification Plant anatomy: Tissues, Roots, Stems, Leaves Transport in plants Plant growth (primary & secondary) The role of Hormones in coordinating growth & response to stimuli For another time… Plant Sexual Reproduction Alternation of Generations

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 from water to land: major adaptations & plant groups

Vascular Plants (tracheophytes) Seedless Vascular Plants Plant Kingdom Byrophytes (aka non-vascular or non-tracheophytes) Mosses Vascular Plants (tracheophytes) 235,000 species 720 species Seedless Vascular Plants Ferns, etc Angiosperms (“vessel seed”) Flowering plants Gymnosperms (“naked seed”) Conifers, etc 65,000 species 170,000 species Monocots (“one cotyledon”) Grasses, orchids, bamboo, palms, lilies, grains Dicots (“two cotyledons”) Most 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…(oak, maple. 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

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? Vascular Tissues XYLEM tissue = Water-conducting: made of tracheids and vessel elements – are both dead, hollow tubes when mature PHLOEM tissue = Food-conducting: made of sieve-tube members (dead when mature) & companion cells (alive when mature) Ferns, gymnosperms and angiosperms have these vascular tissues and are called vascular plants or tracheophytes. Bryophytes (mosses) do not have xylem and phloem and so they are non-vascular (lack specialized transport system).

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. Plasmodesmata are openings in the walls and plasma membranes between plant cells that allow free flow of liquids and dissolved molecules.

From root hairs into xylem – then up 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 (palisade & spongy mesophyll), 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 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 Stem Cross-section Root Cross-section

Vascular cambium makes xylem & phloem while cork cambium makes… cork! Why do trees in temperate climates have annual rings?

Plant hormones coordinate growth, development & responses to stimuli Auxin: directs stem & root growth, responsible for phototropism (growth towards light) and geotropism (growth up away from gravity). Gibberellins: stimulate seed & bud germination, stem elongation, flowering & development of fruit; found in meristems of apical buds, roots, young leaves, embryo. Cytokinins: promote cell division, germination & leaf expansion; determine timing of cellular differentiation (esp. in roots) Ethylene: stimulates the ripening of fruit and the abscission (dropping) of leaves, flowers, and fruits.

The effect of gibberellin treatment on seedless grapes (treated grapes on right).

Plant Tropisms Plant responses to external stimuli Can you guess what these are? Phototropism: Gravitropism: Thigmotropism:

Classic Light/Growth Experiments with Grass Seedlings

MORE Classic Growth Experiments with Grass Seedlings In 1926, F.W. Went extracted the chemical messenger for phototropism, naming it auxin.

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