Plants: Evolution, Classification, Anatomy & Physiology Biology 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 (non-tracheophytes) Mosses Vascular Plants (tracheophytes) Seedless Vascular Plants Ferns, etc Plant Kingdom Gymnosperms (“naked seed”) Conifers, etc Angiosperms (“vessel seed”) Flowering plants 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 …(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 type Collenchyma = support plant in growing parts Sclerenchyma = strengthens & supports in mature wood; ex) fiber & sclereid cells Water-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 Vascular Tissues

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: O 2, CO 2, H 2 O 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 H 2 O... 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 2 nd year Perennials: plants that live many years