Transcript Plant Structure and Growth

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
 food
can be modified
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
 most
meristems that grow laterally in stems
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