principles of biology biol 100c: introductory biology iii secondary growth in plants / leaf...

Principles of Biology

BIOL 100C:BIOL 100C:Introductory Biology IIIIntroductory Biology IIISecondary Growth in Plants /Secondary Growth in Plants /Leaf Structure & AdaptationsLeaf Structure & Adaptations

Dr. P. NarguizianFall 2012

Plant Growth and Development

Leaf structure and function.

The ground tissue in the leaf makes two layers: the palisade mesophyll with long columnar cells just under the upper epidermis, and the spongy mesophyll with more rounded cells below, where photosynthesis occurs.



Figure 7 Cross-sectional anatomy of a leaf.

Leaves are composed of four general tissue layers. The upper epidermis cells make up the outermost layer of the top of the leaf. These cells generate a waxy waterproofing layer called the cuticle.

The palisade mesophyll is a densely packed collection of photosynthetic cells. The spongy mesophyll is a loosely packed collection of photosynthetic cells. The air spaces of the spongy mesophyll create a large surface area for the photosynthetic cells to exchange gases (input of CO2 and output of O2).

The lower epidermis contains the stomata through which gases enter and leave the air spaces of the spongy mesophyll.



Secondary growth is widening or thickening growth.

•Lateral or thickening growth relies on particular lateral meristems: the vascular cambium and the cork cambium.

•Cork cambium produces a tougher epidermal tissue called the periderm.

•All of these additional layers bulk up the plant, providing strength and additional energy reserves.



Figure 8 Secondary and primary growth.

Secondary growth and its interaction with primary growth. New stems and leaves emerge from the apical meristem, exhibiting primary growth. Concurrently, the stem formed during the former year's growth thickens by lateral growth.

The vascular cambium produces cells inward that form secondary xylem; it also produces new cells outward to form secondary phloem.

The outward growth pushes through the epidermis and cortex of last year's primary growth. The parenchyma cells of the cortex become the cork cambium, which produces cork cells of the periderm.



Bark is in part a manifestation of secondary growth.

•Bark consists of periderm and all the other tissues exterior to the vascular cambium including the secondary phloem.

•Removing bark in a complete ring around a tree would kill the tree because the vascular cambium and secondary phloem would be removed.

•Without these, nutrients cannot sustain living roots.



Figure 10 Layers of a tree trunk.

A cross-section through a woody stem. Notice how the bark integrates with the vascular network of the tree, making it inseparable from the functioning plant.



Figure 11 Tree rings.

Annual rings are visible on this cross-section of a Ponderosa pine (Pinus ponderosa Pinaceae).

The growth rings can be used to provide dendrochronologists information regarding past environmental conditions (e.g. drought, pollution, and even fire).



Plant Morphogenesis and Differentiation

Morphogenesis: the establishment of form and function.



What cellular directives drive the development and growth of these primary and secondary tissues?

•Asymmetrical cell division causes polarity: one end of an organism has a different structure and chemistry from the other end.

•The first step in morphogenesis.



Figure 12 Asymmetrical cell division.

The mechanism of asymmetrical division gives rise to differentiation in plant cells.

A meristemoid cell divides asymmetrically into a smaller guard mother cell. The guard mother cell then divides symmetrically to form a second guard cell of equal size.



What determines pattern formation and development of form in the plant?

• If one transplants a mature root or leaf cell in tissue culture, the cells dedifferentiate to meristematic cells. Thus, every cell in a plant has the same genetic blueprint and potential to be any other kind of cell.

• Pattern formation, or the development of form of a plant, depends on the expression of genes in each cell according to its position in the plant and what is happening in nearby cells.



Figure 13 Gene control by neighboring cell types.

Plant cells affect each other, working as a system to regulate growth and development of specialized cells in balance with plant needs.



Plants go through several developmental stages.

• Humans age with hormonal and physical changes affecting our entire body.

• Plants also go through phase changes, but only the daughter cells of the shoot apical meristem change in structure and function.

• Leaf shape and position often change from juvenile phase to adult



Figure 14 Changes in Eucalyptus leaf morphology.

As the leaves of this Eucalyptus globulus plant age, they change shape. Juvenile stage is in upper left, and adult stage is in lower right.



Figure 15 Plant model organism.

Arabidopsis thaliana, a model organism for plant development research, is shown here. The upper image depicts a multileaved plant, with a small cluster of flowers.

The bottom image is a close up of A. thaliana flowers, the characteristic four petals and sepals, and six stamens of the Brassicaceae (Mustard Family).


Plant Structure and Function

Figure 1 Aquatic plant leaves.

This aquatic lily has broad flat waxy leaves that rest on the surface of water, maximizing exposure to sunlight.


Land plant adaptations evolved as a result of variations of water availability on land.

• Shoots growing out of shallow water produce a waxy, waterproof surface, reducing water loss.


Figure 2 Conifer leaves.

These needle-shaped conifer leaves maximize photosynthesis with minimal water loss, due to both their shape (which minimizes surface area/volume), as well as their waxy coating.


Land plant adaptations evolved as a result of variations of water availability on land.

• In general, plants in arid and cold climates grow smaller leaves, whereas plants in warm, moist climates display leaves with larger surface areas.


Figure 3 Tropical leaves.

Trees in tropical forests have access to abundant water and therefore can have large leaves without risking dehydration.



Figure 4 Leaf area index.

The leaf area index equals the ratio of the total upper leaf surface of a plant divided by the surface area of the ground covered (as viewed from above) by the plant.


Leaf arrangement patterns maximize accessibility to light for photosynthesis.

• The leaf area index measures the average degree of coverage by leaves, because leaves are not static structures.


Leaf arrangement patterns maximize accessibility to light for photosynthesis.

• In each species of plant, arrangement of leaves on the stem, called phyllotaxy, occurs in a fixed pattern.


Leaf orientation affects how a plant captures light.

• The way a leaf orients toward the sun also has an effect on the amount of light captured.• Because horizontal leaves capture sunlight more efficiently, growth can occur in low-light

areas such as in the shade.• In brightly lit habitats, most vertical leaves such as seen on grasses provide efficient

orientation.


Figure 6 Climbing ivy.

This ivy plant climbs on the bark of a tree, allowing it to access more sunlight.



Figure 7 Roots.

Roots are an important adaptation to plants living on land. Deep roots are adaptations in plants that need to reach for water below parched soil.



Plant Adaptations Below Ground

•Along with the sugar and gases leaves and stems provide, plants require water and minerals absorbed from soil via their roots.

•Roots also act as a foundation, preventing collapse and easy breakage.



Figure 8 Fibrous roots.

Most monocots, like these grasses, do not grow tall and have fibrous root systems to maximize uptake of water and nutrients near the surface of the soil.



Exceptions to the rule.

• Although the entire root system helps anchor the plant, most of the uptake of water and minerals occurs at the growing tips and associated root hairs.

• Plants require nitrogen in relatively large amounts to construct proteins, nucleic acids, and chlorophyll.

• One way in which plants acquire nitrogen is through a symbiotic relationship with the bacteria in the genus Rhizobium.

• In mycorrhizae, fungal hyphae, or filaments, grow into the roots.• Hyphae create a significantly broader surface area for water and nutrient absorption than a root system that develops alone.

• In return, the fungi take nutrients from the organic molecules produced from photosynthesis.



Figure 9 Symbiotic relationships between microorganisms and plant roots.

Two types of symbiotic relationships:

a)A nitrogen-fixing bacterium (Rhizobium) in the root cortical cell of a leguminous plant.

b)b) More common are mycorrhizae; fungal and root symbiosis.



The Dynamic Symplast• When plants grow on land, the water and nutrients absorbed by the roots must be transported to the photosynthetic cells.

• Phloem, a specialized plant tissue, transports sugars and other solutes such as mineral nutrients, amino acids, and hormones within a plant.

• Xylem, in contrast, transports water and dissolved nutrients.

• Transport in plants takes place through both living and nonliving cells. The combined cytosol of all living cells in a plant is called the symplast.

• The symplast also includes the cytoplasm of plasmodesmata, cytoplasmic channels that connect adjacent living cells.



Figure 10 Plasmodesmata.

Plasmodesmata channels form between adjacent plant cells.


The Dynamic Symplast

• Allow water and nutrients to reach cells seamlessly, enabling plants to address water shortages in a timely fashion.

principles of biology biol 100c: introductory biology iii secondary growth in plants / leaf...

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