Plant Structure & Growth

Plant Structure & Growth

A Few Plant Facts

Plants are the main producers in most terrestrial ecosystems
A plant is adapted to living on land through evolution and through individual response to the environment

The Plant Body

An integration of organs, including (see handout):

Roots
Stems
Leaves
And lateral processes from stems, including
flowers
branches

A Plant’s Root and Shoot Systems are Evolutionary Adaptations

Differentiation of the plant body into an underground root system and an aerial shoot system is an adaptation to terrestrial life
Vascular tissues integrate the parts of the plant body. Water and minerals move up from the roots in the xylem; sugar travels to nonphotosynthetic and actively growing parts via the phloem

Based on differences in anatomy, the angiosperms can be divided into two classes:
Monocots and Dicots

Roots are adapted to:

1. anchor the plant, absorb and conduct water and minerals, and store food (taproots and tuberous fibrous roots)
2. root hairs near the root tips enhance absorption (increase surface area)

The shoot system consists of:

stems, leaves, and flowers
Leaves are attached by their petioles to the nodes of stems

Leaves

Can be modified as:
spines
tendrils
storage structures
bracts
The distinction between floral parts is often arbitrary as these structures are modified leaves

Axillary (lateral) buds are:

buds that are stimulated to grow
they may become flowers or vegetative branches

Stems can be modified as:

stolons
horizontal, with leaves along its length
runners
horizontal, with rosettes at the end
rhizomes
horizontal and underground
bulbs
vertical, underground, and with fleshy leaf bases
tubers
underground, with definite nodes ("eyes")

Plant Tissues

Are arranged into three continuous systems
The dermal tissue system, or epidermis, is an external layer of tightly packed cells that functions in protection
The vascular tissue system, consisting of xylem and phloem, provides transport and support
The ground tissue, made up predominantly of parenchyma, functions in organic synthesis, storage, and support

There are many types of plant cells which are organized into the three major tissue systems

Parenchyma cells
least specialized plant cells
performing general metabolic and storage functions
retain the ability to divide and differentiate into other cell types under certain conditions

Collenchyma cells
support young parts of the plant shoot without restraining growth

Sclerenchyma cells, fibers and sclereids

are supportive cells with thick, lignified secondary walls
many lack protoplasts; thus, at maturity they are unable to elongate
Water-conducting xylem tissue is composed of:
elongated tracheid and vessel element cells that are dead at functional maturity
tracheids are long, thin, tapered cells with lignified secondary walls that function in support and permit water flow through pits

the wider, shorter, and thinner-walled vessel elements have perforation plates through which water flows freely

Phloem is Composed of:

Sieve-tube members which are
living cells that form tubes for the transport of sucrose and other organic nutrients
Each sieve-tube member is connected to one or more companion cells by plasmodesmata

Meristems generate cells for new organs throughout the lifetime of a plant:

Because plants possess permanently embryonic meristems, unlike animals, they can show indeterminate growth

Apical meristems at root tips and shoot buds initiate primary growth (growth in length) and the formation of the three tissue systems

Lateral meristems are responsible for secondary growth (growth in thickness)

Apical meristems extend roots and shoots through the actions of three primary meristems

Primary growth produces the primary plant body, which consists of the three tissue systems
Root tips, protected by the root caps, grow and develop by the activities of cells in the zones of cell division, elongation, and maturation

In the Root (see handout):

the protoderm gives rise to the epidermis, the procambium forms the central vascular stele, and the ground meristem produces the ground tissue of the cortex
subsequent lateral roots arise from the pericycle of the stele

In the Shoot (see handout):

Elongation is a result of the "products" of the dome-shaped apical meristem (and its three primary meristems) at the top of the terminal bud

leaf primordia arise from the sides of the apical dome

axillary buds arise from residual islands of meristematic cells at the bases of leaf primordia
The vascular tissue of stems runs in vascular bundles surrounded by ground tissue in characteristic patterns that differ between monocots and dicots
form a central stele in many roots

Leaves:

are covered with a waxy epidermis (see overhead)
pairs of guard cells flank openings called stomata (gas exchange and transpiration)
the ground tissue, or mesophyll (between the upper and lower epidermis) consists mainly of parenchyma cells with chloroplasts for photosynthesis
a leaf trace (a vascular strand) connects the veins of the leaf with the vascular tissue of the stem

The Shoot

is a series of modules, each consisting of a node with leaves, an axillary bud, and an internode
phase changes in the development of the shoot tip alter the morphology of modules
phase changes refer to the development of the terminal bud through time as well as the development of any lateral buds (also through time)

Lateral meristems add girth to stems and roots

Secondary growth produces the secondary plant body, the tissues that cause an increase in diameter
The increase in the girth of stems and roots is due to secondary production of new cells by the vascular cambium and the cork cambium, two lateral meristems
The vascular cambium, a continuous cylinder of meristematic cells, produces secondary xylem internally and secondary phloem externally

The cork cambium, a meristematic cylinder in the outer cortex of the stem, produces:

waxy cork cells externally and

phelloderm (a parenchyma) internally
The cork cambium, phelloderm, and cork make up the the periderm, which replaces the epidermis that sloughs off during secondary growth
Secondary phloem gives rise to new cork cambium after the original cortex is shed
Bark consists of phloem plus periderm

In roots, the vascular cambium arises between the xylem and phloem of the stele and functions similarly to that in stems

Cork cambium, produced from the pericycle of the stele, forms the periderm that replaces …
cortex and epidermis

Our Focus

The "production" of the plant body which is composed of (see handout):

The 3 primary tissues (epidermis, vascular tissue, and ground tissue) are derived from:
The 3 primary meristems (protoderm, procambium, and ground meristem); in biennial (+) dicots, growth in diameter is achieved through:
The lateral (secondary) meristems (vascular cambium and cork cambium), which give rise to:
The secondary tissues (secondary xylem & phloem AND cork and phelloderm)

Our Focus (continued)

The "structure" of the apical meristems
shoot
root
The mechanisms for apical meristem tissue production
The "structure" of lateral meristems
vascular cambium
cork cambium
The mechanisms for lateral meristem tissue production

Our Focus (continued)

The connection between structure and function of leaves

The "Structure" of the Apical Meristems

Shoot Apical Meristem (SAM)
The SAM gives rise to:
protoderm
procambium
ground meristem
Root Apical Meristem (RAM)
The RAM gives rise to:
protoderm
procambium
ground meristem

The Mechanisms for Shoot Apical Meristem Tissue Production

Organization of the SAM
Tunica
an outer region of the SAM that gives rise to the protoderm through anticlinal divisions
Corpus
an inner mass of cells in the SAM that gives rise to the procambium and ground meristems through periclinal divisions

The Three Primary Meristems Give Rise to the Three Primary Tissues - Shoots

Protoderm
gives rise to the epidermis

see overhead
Procambium
gives rise to bundles of vascular tissue [note the difference between monocots and dicots]
see overhead
Ground meristem
gives rise to cortex and pith in dicots (undifferentiated ground meristem in monocots)
see overhead

The Three Primary Meristems Give Rise to the Three Primary Tissues - Roots

Protoderm
gives rise to the epidermis

see overhead
Procambium
gives rise to a stele (central cylinder of vascular tissue) [note the difference between monocots and dicots]
see overhead
Ground meristem
gives rise to cortex (and pith in monocots)

endodermis

pericycle

lateral roots

see overhead

The "Structure" of Lateral Meristems

Vascular Cambium
Cork Cambium

The Mechanisms for Lateral Meristem Tissue Production

Vascular Cambium (vc) is composed of (see Figure 35.23):
Fasicular Cambium
portion of vc within a vascular bundle
meristematic cells are called fusiform initials

gives rise to secondary xylem & phloem
Interfasicular Cambium
portion of vc between vascular bundles
meristematic cells are called ray initials

gives rise to rays (both xylem rays and phloem rays)

Cork Cambium (cc)

forms within the primary cortex of woody dicots
Initials within the cc give rise to phelloderm to the inside and cork to the outside
Periderm
phelloderm + cc + cork
What is bark?
Periderm + secondary phloem (everything outside the vc)

The Secondary Tissues

Secondary xylem
Phloem
Cork
Phelloderm

Leaves

How is structure related to function?
Photosynthesis requires … ?
Carbon dioxide
Stomata are the "pores " through which gas exchange occurs
Transpiration involves the loss of water, especially through stomata
Protection from water loss?
Epidermis is covered by cuticle
Sunken stomata
Hairs on leaf surfaces
Adaptations for photosynthetic efficiency?
Mesophyll layers