Plant Structure and Growth

Chapter 35

Objectives List the differences between dicotyledons and

monocotyledons Describe the basic structure of all plants Describe how the structure of plants is an adaption

to the basic function of plants in terrestrial environments

Name the six types of cells and three types of tissue systems found in plants and describe their distinguishing characteristics

Differentiate between primary and secondary growth, and describe where and how each kind of growth occurs

Introduction

Angiosperms are largest group of plants make up 90% of world’s plant species two major groups of angiosperms

monocotyledons• include orchids, bamboos, palms, lillies and grasses• distinguishing characteristics include

– single seed leaf; cotyledon– leaves usually parallel-veined– scattered vascular bundles in stems– floral parts in multiples of three– fibrous root system

dicotyledons• includes most angiosperms including most shrubs and

trees (except conifers) and many herbaceous plants

• distinguishing characteristics include

– two cotyledons

– net-veined leaves

– vascular bundles in ring in stems

– floral parts in multiples of four or five

– taproot system

Plant Structure

Plant body consists of roots and shoots allows plants to function in terrestrial

environment take up water and minerals from soil absorb light take in CO2 from air create plant bodies from molecules assembled from

these raw materials and products of photosynthesis

root system anchors plant absorbs and transports minerals, water and stores

food ultimate site of absorption is root hair

• outgrowth of epidermal cells

• increases absorptive surface area

shoot system consists of supporting stems, photosynthetic leaves

and reproductive structures composed of

• nodes-point where leaves, flowers and other stems attached

• internodes-stem between nodes

• leaves composed of photosynthetic blades and short stalks (petioles) that join blades to nodes

buds undeveloped shoots

• contain potential nodes, internodes and leaves

• two types

– terminal bud at plant apex; source of growth in height

– axillary bud in angle of petiole and stem; usually dormant but can produce new branches

apical dominance results from release of hormones from terminal buds

• inhibits growth of axillary buds

• removal of terminal bud stimulates development of axillary buds

– basis for pruning

Many plants have modified roots and shoots modified tap roots of some dicots used for food storage

stored as starch• examples-root crops

stems can be modified for several purposes asexual reproduction

• runners-strawberries

food storage• rhizomes-irises• tubers-potatoes

leaves can be modified food storage

• leaf bases of celery

grasping and climbing• tendrils of cucumbers

protection• spines of cactus

Plant cells and tissues diverse in structure and function cells have unique features

photosynthetic and contain chloroplasts often have large central vacuole; maintain turgor bounded by cell wall composed of cellulose many cells have additional secondary wall hardened with

lignin plasmodesmata provide continuous cytoplasmic link

between adjacent cells

six types of plant cell; based on wall morphology and chemistry, shape and function parenchyma

• abundant and unspecialized; primary cell walls

• food storage, photosynthesis and aerobic respiration

collenchyma• similar to parenchyma but have thicker primary wall

• provide support for young growing parts of plant

sclerenchyma• have rigid cell walls hardened with lignin

• provide support and protection

– seed coats

– gritty texture of pear

vessel elements and tracheids• found in xylem

• elongated and secondary wall thickening; dead and open ended when functioning as water conducting cells

• connected end-to-end

• tracheids

– tapered ends

– covered with open pits

• vessel elements

– wider and shorter; completely open ends

sieve-tube members• found in phloem

• relatively thin primary walls, no secondary wall; alive but lack nucleus and ribosomes when functioning

• contain numerous pits with plasmodesmata

• associated with at least one companion cell

– provides nucleus functions for sieve-tube member

Three tissues make up plant body epidermis

composed of single, surrounding layer of cells first defense against infection and damage

vascular tissue composed of xylem and phloem conducts water and nutrients through plant

ground tissue fills space between epidermis and vascular tissue composed mainly of parenchyma functions include photosynthesis, storage and

support

each system continuous from organ to organ

roots surrounded by epidermal cells with root hairs; no

cuticle ground tissue (cortex) conducts material from root

surface to central vascular tissue inner layer of cortex (endodermis) forms selective

barrier; regulates flow into vascular tissue

stems epidermal cells covered by waxy layer-cuticle dicots-vascular tissue bundles in outer ring of

ground tissue cortex surrounding parenchyma pith monocots-vascular tissue bundles scattered in

uniform ground tissue

leaves also have cuticle lower epidermis includes pores (stomata)

surrounded by guard cells-gas exchange ground tissue arranged in two mesophyll layers

• lower loose layer (spongy mesophyll) for gas exchange

• upper compact layer (pallisade mesophyll) for photosynthesis

branches of vascular tissue enter leaf and provide transport to and from photosynthetic cells

Plant Growth

Primary growth lengthens roots and shoots indeterminate growth-grow during entire life three seasonal growth patterns

annuals-complete life cycle in one year biennials-complete life cycle in two years perennials-live and reproduce for many years

indeterminate growth results from presence of meristems unspecialized cells that continue to divide apical meristems at root and shoot tips and in

axillary buds differentiation controlled by master control genes

(homeotic genes)

apical meristem in root tip divides cells downwards root cap protects meristem; abraided by soil other cells grow upward, forming three rings of

tissue-become epidermis, cortex and vascular cylinder

• above meristem cells elongate; force root tip down

• above this region, cells differentiate

apical meristem of shoot forms three downward-forming cylinders of embryonic tissue contains zones of elongation and differentiation some meristem cells remain in lateral position

• form meristem of axillary buds

Secondary growth increases girth of woody plants involves meristems that grow laterally in stems

most evident in trees, shrubs and vines

vascular cambium-cylindrical meristem develops from parenchyma cells between xylem and

phloem of shoots cells dividing inwards form new secondary xylem

outside primary xylem cells dividing outwards add secondary phloem inside

primary phloem

secondary xylem cells larger during favorable growth periods and smaller at other times annual growth rings

new layers of phloem do not accumulate sloughed off in bark at same rate produced in secondary phloem, meristematic cells (cork

cambium) produce cork cells• dead when mature

• thick, waxy walls-protect stem surface

wood divided into two layers heartwood-nonfunctioning xylem, plugged with

resin• acts as endoskeleton; strong, rigid but flexible core

sapwood-functioning secondary xylem wood rays are collections of parenchyma cells

forming connections between heartwood and sapwood