Untitled

Plant Organs: Stems

Functions of Stems

  • Support leaves and reproductive structures

    • Upright position for maximum light absorption

    • Reproductive structures accessible for pollination and seed dispersal

  • Provide internal transport

    • Conduct water, minerals, and carbohydrates

    • Not the only organs for material conduction

Stem Structure and Function

  • Vegetative vascular plants have roots, stems, and leaves

  • Stems link roots to leaves and can be aerial or underground

  • Stems can be herbaceous or woody, with varied forms

  • Three main functions of stems:

    • Support leaves and reproductive structures

    • Provide internal transport

    • Conduct water, minerals, and carbohydrates

Pine Stems

  • Eastern white pine highly valued for its straight, tall stem

  • Wood suitable for various constructions and items

  • Overlogging led to the decline of virgin white pine forests

  • Pine stems used for lumber, paper, turpentine, and rosin

  • Pines grown for ornamental purposes and environmental benefits

Stem Variation

  • Stems range from vines to massive tree trunks

  • Herbaceous stems consist of soft tissues, while woody stems have hard tissues

  • Stems conduct water, minerals, and carbohydrates within plants

Stem Examples

  • Morning glory with trailing, twining stem for covering fences

  • Baobab trees with massive trunks adapted for water and starch storage

Conclusion

  • Stems play crucial roles in plant structure and function

  • Understanding stem diversity and functions is essential in plant biology.

Page 4: Stem Growth and Structure

  • Stems play a crucial role in plants by supporting, conducting, and producing new tissue.

    • Primary growth occurs at apical meristems, increasing plant length.

    • Secondary growth, due to lateral meristems, increases plant girth.

  • Woody plants have both primary and secondary growth, while herbaceous plants have only primary growth.

  • Stems can be modified for asexual reproduction, food storage, or photosynthesis.

  • All stems have buds, including terminal buds and axillary buds.

  • Woody twigs of deciduous trees show stem structures, with bud scale scars indicating age.

  • The external structure of all stems includes nodes, internodes, and leaf scars.

Page 5: Herbaceous Eudicots and Monocots Stems

  • Stems of herbaceous eudicots and monocots have an outer protective covering, ground tissue, and vascular tissues.

  • Herbaceous eudicot stems have vascular bundles arranged in a circle around a central pith.

  • The epidermis provides protection, and a cuticle reduces water loss.

  • Vascular bundles in eudicot stems consist of xylem and phloem, with phloem fibers for support.

  • Lenticels on woody twigs allow gas exchange and aid in plant identification.

Page 6: Herbaceous Eudicot Stem Anatomy

  • Cortex in Herbaceous Eudicot Stem:

    • Contains parenchyma, collenchyma, and sclerenchyma cells.

    • Functions include photosynthesis, storage, and support.

    • Parenchyma cells conduct photosynthesis and store starch.

    • Collenchyma and sclerenchyma provide structural support.

  • Vascular Tissues:

    • Vascular bundles in a circle, extending throughout the stem.

    • Xylem transports water and minerals, phloem transports carbohydrates.

    • Vascular cambium responsible for secondary growth.

  • Stems Strength:

    • Stems stronger than roots due to tracheids, vessel elements, and fibers.

    • Phloem fiber cap in some herbaceous eudicot stems for strength.

  • Monocot Stems:

    • Vascular bundles scattered, enclosed in bundle sheath of sclerenchyma.

    • No distinct cortex and pith, no lateral meristems for secondary growth.

Page 7: Monocot Stem and Woody Eudicot Stem Development

  • Monocot Stem:

    • Vascular bundles scattered, enclosed in sclerenchyma bundle sheath.

    • No distinct cortex and pith, no secondary growth with lateral meristems.

  • Woody Eudicot Stem Development:

    • Primary tissues organized like herbaceous eudicot stems.

    • Vascular cambium produces secondary xylem (wood) and phloem (inner bark).

    • Cork cambium produces cork cells and parenchyma, forming periderm.

Summary Tables

  • Table 7-1: Differences between Herbaceous Eudicot Roots and Stems:

    • Roots vs. Stems variations in nodes, leaves, photosynthesis, pith, and more.

  • Table 7-2: Relationships in a Woody Eudicot Stem:

    • Apical and lateral meristems, primary and secondary tissues in woody plants.

The transcript discusses the anatomy of herbaceous eudicot stems, monocot stems, and woody eudicot stem development. It highlights the functions of cortex, vascular tissues, and strength components in stems. Monocot stems lack secondary growth mechanisms, while woody eudicot stems undergo secondary growth with vascular and cork cambium. Tables summarize differences between roots and stems, as well as meristem and tissue relationships in woody eudicot stems.

Page 8

  • Vascular Cambium and its Functions

    • Cells between vascular bundles can divide.

    • These cells connect to vascular cambium cells to form a complete ring.

    • Vascular cambium produces secondary xylem and secondary phloem.

    • Cork cambium produces cork cells and cork parenchyma for the outer bark.

  • Development of Secondary Growth

    • Vascular cambium divides to form secondary xylem on the inside and secondary phloem on the outside.

    • Vascular cambium arises in the parenchyma between vascular bundles.

    • Woody stems show more secondary xylem production than secondary phloem.

Page 9

  • Vascular Cambium Division

    • Dividing vascular cambium cells become secondary xylem or secondary phloem.

    • Vascular cambium is a thin layer between wood and inner bark.

    • One cell remains meristematic while the other develops into mature tissue.

  • Changes in Stem Tissues

    • Secondary growth changes the orientation of primary tissues.

    • Secondary xylem and phloem are laid down between primary xylem and phloem.

    • Vascular cambium increases in cells through occasional radial divisions.

Page 10

  • Stem Cross-Section

    • Annual rings of secondary xylem and secondary phloem are visible.

    • Vascular cambium is located between secondary xylem and phloem.

    • Cork cambium, cortex, and epidermis remnants are present in the outer layers.

  • Three-Year-Old Stem

    • Cross-section shows the arrangement of primary and secondary tissues.

    • Vascular cambium separates secondary xylem and phloem.

The transcript discusses the functions of vascular cambium in producing secondary xylem and phloem, the development of secondary growth in woody stems, the division of vascular cambium cells into secondary tissues, changes in stem tissues during secondary growth, and the structure of a three-year-old stem in cross-section.

Page 11

  • Secondary Growth in Plants

    • Primary xylem and phloem are separated from each other during the formation of secondary tissues.

    • Secondary xylem conducts water and minerals from roots to leaves, containing various cell types like tracheids, vessel elements, parenchyma cells, and fibers.

    • Secondary phloem transports carbohydrates vertically within the plant.

    • Lateral movement of materials is facilitated by rays, chains of parenchyma cells extending from the center of the stem.

    • Cork cambium produces periderm, replacing the epidermis for protection.

  • Cork Cambium and Periderm

    • Cork cambium produces cork cells with waterproofed walls to protect the stem.

    • Cork parenchyma stores water and food, thinner than the cork cell layer.

    • Lenticels in the cork allow gas exchange as the stem thickens.

    • Cork oak bark is commercially important for various products like bottle stoppers, flooring, and insulation.

Page 12

  • Wood Characteristics

    • Sapwood, the younger wood closest to the bark, conducts water and minerals.

    • Heartwood, older wood in the trunk's center, is denser and provides structural support.

    • Heartwood is plugged with pigments and other materials, no longer functioning in conduction.

    • Heartwood is more resistant to decay and has a brownish-red color.

  • Variation in Bark

    • Different tree species have varied outer bark textures like fissured, rough, scaly, or peeling.

    • Cork cambium divides to form new tissues inside and outside, with cork cells protecting against mechanical damage and environmental factors.

    • Heartwood and sapwood have distinct functions and appearances in wood.

Page 13: Importance of Wood

  • Wood Types: Hardwood from flowering plants, softwood from conifers

    • Conifer wood lacks fibers and vessel elements, only has tracheids

    • Balsa tree, a flowering plant, has extremely soft wood

  • Common Wood Products: Lumber, paper pulp

    • Lumber is air-dried, graded, and sorted for various products like furniture, bats, toys

    • Paper pulp mainly comes from conifers, processed in sawmills

Page 14: Woody Plant Growth

  • Annual Rings: Found in temperate climate plants with distinct growing and dormant periods

    • Rings indicate age, but not reliable in tropical trees due to environmental factors

  • Stem Structure: Heartwood and sapwood in woody plant stems

    • Sapwood functions in water and mineral conduction

  • Lenticel Structure: Cross-section of a lenticel in a calico flower stem

    • Includes cork cambium, cork cells, and cork parenchyma

By understanding the types of wood and their uses, as well as the growth patterns in woody plants, we can appreciate the significance of wood in various industries and ecosystems.

Plant Organs: Stems

  • Annual Rings in Woody Stems

    • Differences in cell size and cell-wall thickness create variations in secondary xylem.

    • Springwood has thin-walled, large-diameter conducting cells, while late summerwood has thicker-walled, narrower cells.

    • Annual rings provide information about past climates.

    • Wood appearance varies based on cutting direction: cross section, tangential section, and radial section.

Tree-Ring Analysis

  • Process of Science

    • Counting annual rings determines a tree's age and reflects local weather conditions.

    • Tree-ring analysis helps study past climates, environmental effects, and natural disasters.

    • Tree rings indicate temperature variations and global climate patterns.

    • Scientists use tree-ring data to understand human influence on climate change.

Forests and Deforestation

  • Ecological Importance of Forests

    • Forests play a crucial role in the global carbon cycle through photosynthesis and oxygen release.

    • Tree roots prevent erosion and regulate water flow, benefiting wildlife habitats.

  • Deforestation

    • Deforestation poses a significant threat to world forests.

    • Removal of forests for agriculture or other purposes impacts ecosystems and biodiversity.

Tree-Ring Dating and Deforestation

  • Tree-Ring Dating

    • Scientists develop master chronologies to determine the age of wood samples accurately.

    • Matching and overlapping older wood sections help extend dating back in time.

  • Deforestation

    • Deforestation involves the temporary or permanent clearance of large forest areas for various uses.

    • Presence of knots in wood affects its commercial value, except for ornamental purposes.

Page 17

Deforestation

  • Deforestation leads to the replacement of forests with agricultural and non-forest land.

    • Only about one-third of harvested trees are replaced with fast-growing seedlings.

  • Particularly acute in humid tropics.

  • Impact on the environment:

    • Decreased soil fertility and increased soil erosion.

    • Altered water flow leading to floods and droughts.

    • Extinction of plant and animal species.

Plant Stem Anatomy

  • Annual ring structure in a Tilia stem:

    • Shows growth increments with differences in vessel elements between springwood and late summerwood.

  • Wood block sections:

    • Cross, tangential, and radial sections.

    • Each section has distinctive features like rays and annual rings.

Page 18

Tropical Rain Forest Deforestation

  • Tropical rain forests found in Central and South America, Africa, and Southeast Asia.

  • Main causes of deforestation:

    • Subsistence agriculture, commercial logging, and cattle ranching.

  • Subsistence agriculture:

    • Accounts for over half of tropical rainforest loss.

    • Involves slash-and-burn technique leading to soil degradation.

  • Impact of deforestation:

    • Threatens native people and their cultures.

    • Decreases soil fertility and increases soil erosion.

    • Alters water flow patterns causing floods and droughts.

    • Leads to extinction of plant and animal species.

Global Impact

  • Over 200 million subsistence farmers rely on slash-and-burn agriculture.

  • Forest availability has decreased by half in the last 50 years.

  • Distribution of tropical rain forests in Central and South America, Africa, and Southeast Asia.

Note: Deforestation in tropical rain forests has severe consequences on the environment, native populations, and global biodiversity. Sustainable agricultural practices are crucial to mitigate these impacts and preserve the forests for future generations.

Page 19

Vines

  • Vines are weak-stemmed plants that rely on other plants for support.

    • They grow rapidly by ascending through the forest canopy.

  • Vines have adaptations for climbing such as tendrils, adhesive pads, and twining stems.

  • Tropical rain forests have a high diversity of vines, while temperate forests have fewer.

  • Vines in tropical rain forests are less affected by droughts and fires compared to temperate forests.

  • Asexual reproduction in plants involves the formation of offspring that are genetically similar to the parent.

  • Flowering plants have various methods of asexual reproduction using modified stems like rhizomes, tubers, bulbs, corms, and stolons.

Page 20

Modified Stems

  • Rhizomes are horizontal underground stems used for storage and reproduction.

  • Tubers are thickened ends of rhizomes for food storage.

  • Bulbs are rounded underground buds with fleshy leaves.

  • Corms are short underground stems covered by papery scales.

  • Stolons are horizontal aerial stems that produce new plants at nodes.

  • Different plants like irises, potatoes, lilies, and strawberries reproduce asexually using these modified stems.

Page 21

Functions of Stems

  • Support leaves and reproductive structures

    • Conduct water, dissolved minerals, and carbohydrates

    • Produce new living tissues at apical and lateral meristems

Herbaceous Stem Tissues

  • Epidermis, vascular tissue, cortex, and pith

    • Epidermis: protective outer layer with a water-conserving cuticle

    • Xylem conducts water, phloem conducts carbohydrates

    • Cortex, pith, and ground tissue primarily for storage

Herbaceous Eudicot vs. Monocot Stem Structures

  • Eudicot: vascular bundles in a circle, distinct cortex and pith

  • Monocot: scattered vascular bundles, ground tissue instead of distinct cortex and pith

Stems vs. Roots

  • Stems have nodes, internodes, leaves, and buds

  • Roots have root caps, root hairs, endodermis, and pericycle

Transition to Secondary Growth in Woody Stem

  • Woody eudicots and conifers undergo secondary growth

  • Vascular cambium produces secondary xylem (wood) and secondary phloem (inner bark)

  • Cork cambium produces cork parenchyma and cork cells

Specialized Stems for Asexual Reproduction

  • Rhizome, tuber, bulb, corm, stolons

    • Rhizome: horizontal underground stem

    • Tuber: underground stem for storing food

    • Bulb: underground bud with fleshy storage leaves

    • Corm: underground stem resembling a bulb

    • Stolons: aerial horizontal stems with long internodes

Page 22

Herbaceous Eudicot Stem Tissues

  • Label tissues: epidermis, vascular tissue, cortex, pith

  • Functions: protection, water and nutrient transport, storage

Monocot Stem Tissues

  • Label tissues: scattered vascular bundles, ground tissue

  • Functions: water and nutrient transport, support

Thought Questions

  • Possibility of tracheid and sieve-tube element division during secondary growth

  • Lack of annual rings in tropical tree wood

  • Desirability of hardwood over softwood for furniture making

  • Differentiating between root and underground stem without a microscope

  • Proposal