9.1 - Transport in the Xylem of Plants

Xylem

Function: 

• The primary function of xylem is to transport water and dissolved minerals from the roots to the rest of the plant. 

  • This process, called transpiration, is essential for maintaining the plant's hydration and providing the necessary raw materials for photosynthesis. 

• In addition to water, xylem also transports essential mineral nutrients, such as potassium, calcium, and magnesium, from the soil to the various parts of the plant where they are needed for growth and metabolic processes. 

• Xylem also provides structural support to the plant. 

  • The lignin-containing secondary cell walls in xylem cells give them strength and rigidity, helping the plant maintain an upright position. 

• Xylem can serve as a storage site for some substances, although its primary role is transportation. 

  • For example, some xylem cells can store starch and other substances during certain stages of their development.

Locations and Shapes: 

• Often “x” or star-shaped or in a ring in young roots, toward the inside of phloem in vascular bundles in stems/roots, on top of phloem in leaf veins

  • Roots: In roots, xylem is located in the center, forming a structure called the "stele." Xylem cells in the root help transport water and minerals from the soil into the rest of the plant.

  • Stems: In stems, xylem is typically found in the inner region of the stem, just outside the pith (the central part of the stem). This arrangement provides support and allows for the transport of water and nutrients up the stem.

  • Leaves: Leaves contain specialized xylem tissue known as "veins." These veins distribute water to the leaf cells, where it is used in photosynthesis and other metabolic processes.

Cells found in this tissue:

Vessel Elements/Members:

• Dead at maturity--function as hollow dead cells

• Large in diameter compared to other cells--large volume transport of water and solutes

• Vessel elements have perforated end walls, which are known as vessel end plates. 

  • These end plates are essentially open pores that allow for easy passage of water and dissolved minerals from one vessel element to another, creating a continuous and efficient water-conducting system.

• They have pits that allow for side-to-side conduction. 

• They play a crucial role in the upward movement of water and minerals from the roots to the leaves and other parts of the plant. 

  • The interconnected vessel members create a low-resistance pathway for water to flow through.

• 

Tracheids

• They are dead at maturity--also function as dead hollow conducting cells

• They are smaller in diameter than vessel elements but more elongate with tapered ends

• They develop end-on-end and connected at ends by pits and also have pits for side-to-side conduction. 

• They contribute to the overall strength and rigidity of the plant. 

  • Their lignified walls provide mechanical support, helping the plant stand upright and resist the forces of wind and gravity.

• Tracheids have various structural adaptations to optimize water transport. 

  • Their end walls contain small pores, called pits, which allow water to move from one tracheid to another. 

  • Tracheids lack the perforations or "vessel elements" found in another type of xylem cell called vessels, which allows them to transport water more slowly but with greater resistance to embolisms (air blockages).

Fibers

• Fiber cells in xylem are long, slender cells with thick, lignified cell walls. 

  • The primary function of these cells is to provide mechanical support and strength to the plant. 

  • They are often found in the secondary xylem (wood) of woody plants and contribute to the plant's overall structural integrity

• Fiber cells have thick cell walls that are heavily lignified. 

  • Lignin is a complex polymer that provides rigidity and durability to the cell walls, making them resistant to decay and mechanical stress. 

  • It is what gives wood its strength and hardness.

• Fiber cells are typically found in the outermost part of the xylem, which is known as the xylem fibers or the wood. 

  • They are often closely associated with vessel elements and tracheids, which are the primary water-conducting cells of the xylem.

Parenchyma

• Parenchyma cells are generally thin-walled and living at maturity. 

  • They have a simple structure with a primary cell wall, and they lack the secondary cell wall characteristic of other xylem cell types like vessel elements and fibers.

• Parenchyma cells in the xylem play a role in storage and metabolism. 

  • They can store starch, oils, and other substances, serving as a reservoir of nutrients and energy for the plant.

• Parenchyma cells have the ability to divide and differentiate, making them important for tissue repair and regeneration in the xylem. 

  • When a plant undergoes stress, damage, or growth, parenchyma cells can divide and give rise to other xylem cell types.

  • A type of parenchyma that has sclerified, which means it has developed a secondary wall and died at maturity.

9.2 - Transport in the Phloem of Plants

Phloem

Function: 

• Phloem primarily transports sugars (mainly sucrose) produced during photosynthesis in the leaves to other parts of the plant, including roots, stems, and developing fruits and seeds. 

  • This transport allows plants to use the energy stored in sugars for growth and metabolism.

• In addition to sugars, phloem also transports various other organic compounds, such as amino acids, hormones, and growth regulators, to support different metabolic processes and growth in various parts of the plant.

• Phloem can also function as a storage tissue for carbohydrates and other compounds. 

  • Some plants store excess sugars as starch in phloem cells.

• Phloem enables long-distance transport of nutrients and energy, making it essential for the overall health and functioning of plants.

Location and Shapes 

Phloem is found in various parts of the plant, primarily the stems and roots. It is also present in leaves, where photosynthesis occurs and where the sugars are initially produced.

• Phloem tissue forms a network throughout the plant, connecting all the different parts. 

  • It often occurs in close association with xylem, which transports water and minerals.

• It is usually outside of xylem in vascular bundles in stem and root

  • and below xylem leaf veins

Cells found in this tissue:

Sieve tube elements/members (STE)

• Specialized parenchyma that lacks a nucleus and many other cellular components

• Develop end-on-end and connected by specialized sieve plates with a high density of plasmodesmata

• Contents under positive (albeit low) pressure

Companion cells

• Develop adjacent to sieve elements--one or more is located adjacent to each STE. Several plasmodesmatal connections between them

• Have a nucleus--evidence suggests that these cells help coordinate STE function

• Not directly involved in large-volume transport

Fibers

• Support and protection function

• May be clustered together in bundles

• Economically important in some species

Parenchyma

• Phloem parenchyma cells are living cells and are one of the four main types of cells found in the phloem tissue. 

  • The other three cell types in phloem are sieve elements, companion cells, and fibers.

• They are typically located both inside and outside the phloem vessels (sieve tubes and companion cells) and are dispersed among these elements.

• The structure of phloem parenchyma cells can vary among plant species. 

  • They may have thin cell walls and large central vacuoles, allowing for storage and maintaining turgor pressure.

• These cells are interconnected, allowing for the exchange of nutrients and signaling molecules.

• Phloem parenchyma cells are involved in the loading of sugars into the phloem for long-distance transport. 

  • In some plants, they assist in actively transporting sugars from photosynthetic tissues into the phloem.

• During unloading, phloem parenchyma cells can receive and distribute nutrients to the surrounding plant tissues.

9.3 - Growth in Plants

Seeds

Seeds are produced by two major groups of plants: Angiosperms and Gymnosperms. 

Each seed is the result of one fertilized ovule from the parent plant.

Some components of the seed:

• Seed coat: or testa

  • Role: protection, composed of parenchyma and scelerids

  • dies and shed at maturity

• Embryo (a complex, multipartite structure)

  • contains all sites cells the plant will need to initiate growth

• Embryonic axis: Shoot and root structures

  • Shoot structures: plumule (leaves), epicotyl and hypocotyl (stem)

  • Root structures: radicle (will become root) 

• Cotyledons: “seed leaves”, one or more

  • may be primary location of energy storage used in germination

• Endosperm: nutrient-rich storage

  • corn/cereal grains→ high mass endosperm

  

Dormancy and Germination

Dormancy is defined as a period of growth inactivity.

Certain seeds remain dormant and viable for an extended period of time—others don’t.

• cool storage, dark, dry, little/low air circulation

Different species have different requirements to break dormancy and germinate. These requirements may be environmental and/or physiological.

• temperature → period of cold followed by warm (winter)

  • day length change

• injury/abrasion to seed coat → scarification

• smoke compound/heat from low grade fire (ex. Pimus sarrotina, lodge pine, sand pine)

• hormone levels

Germination is defined as the process of initiating embryonic growth of the emergence of shoot and root

Imbibition: water absorption through pores in seed coat → seed swelling as it “drinks in”

Root and Shoot Primary Growth

Meristems (Meristo= to divide)

Characteristics of meristems or meristematic cells of roots and shoots

• small

• thin primary walls

• mostly nucleus by volume

Meristem Types

• Apical (tip ends)—Apical meristems contribute to primary growth—growth in length or height or initiation of new organs or branches.
  Examples of apical meristems:

  • shoot apical meristems (SAM) → found in buds

  • root apical meristems (RAM)

  

Primary meristems are derived from shoot and root apical meristems. There are three primary meristems that correspond to the three tissue systems:

Apical Meristem

• → Protoderm → Dermal

• → Procambium → Vascular

• → Ground → Ground

In roots, root apical meristems at the end of each root branch produce a protective structure over the meristem called a root cap.

Shoot apical meristems have distinct appearances, depending on the plant group, but do not produce a cap. Young, developing leaves cover the meristem instead.

• Intercalary—Intercalary meristems occur between 2 mature tissues. These meristems frequently occur at the bases of grass leaf blades, contributing to leaf elongation from the base. The growth resulting from intercalary meristems is considered primary growth.

• Lateral—Lateral meristems contribute to secondary growth—growth in width or girth.

9.4 - Reproduction in Plants

Reproduction in plants is a fundamental process that allows them to propagate and ensure the survival of their species. It encompasses a variety of mechanisms, strategies, and adaptations that have evolved over time. Here, we will explore some key aspects of plant reproduction.

Asexual Reproduction:

Vegetative Reproduction: Many plants have the ability to reproduce asexually by producing new individuals from vegetative parts, such as stems, leaves, or roots. This process is common in succulent plants like cacti, where a segment of the stem can grow into a new plant when detached.

Runners and Rhizomes: Some plants, like strawberries, send out runners or horizontal stems (rhizomes) that can develop into new plants. These runners establish roots and grow independently, creating a clone of the parent plant.

Rhizome: horizontal underground stems

• short internodes

• thickened for storage

Stolon: horizontal stem aboveground or underground

• longer internodes

• smaller in diameter

• some produce asexual plantlets

Tuber: part of stolen that is modified for storage (e.g. potato).

Sexual Reproduction:

Flower Anatomy: In sexually reproducing plants, flowers play a central role. Flowers are complex structures with both male and female reproductive organs. The male part, the stamen, typically consists of the anther (where pollen is produced) and the filament (a stalk that supports the anther). The female part, the pistil, includes the stigma (where pollen lands), the style (a tube connecting the stigma to the ovary), and the ovary (containing ovules).

Pollination: To achieve fertilization, pollen must be transferred from the anther to the stigma. This process can occur through various mechanisms, including wind, insects, birds, or other animals. The evolution of different pollination strategies has led to a wide variety of flower shapes, colors, and scents.

Double Fertilization: In angiosperms (flowering plants), a unique reproductive feature is double fertilization. One sperm cell fuses with an egg cell to form a zygote, which develops into the embryo. The other sperm cell combines with two polar nuclei to create a triploid cell, which develops into the endosperm. The endosperm serves as a source of nourishment for the developing embryo.

Seed Formation: After fertilization, the ovule develops into a seed, containing the embryo, endosperm, and a protective seed coat. The mature seed is capable of withstanding unfavorable conditions and waiting for the right time to germinate.

Fruit Development: The ovary of the flower matures into a fruit, which protects and aids in the dispersal of seeds. Fruits come in a wide range of forms and can be dispersed by wind, water, animals, or even explosively.

Germination:

Imbibition: The germination process begins with the uptake of water through the seed coat. This initiates metabolic processes within the seed, leading to its swelling and softening

Emergence of the Radicle: The radicle, the embryonic root, is the first structure to emerge from the seed. It anchors the plant and starts to absorb water and nutrients from the soil.

Shoot Growth: The plumule, which contains the embryonic shoot, then emerges from the seed. It develops into the stem and leaves of the new plant.

Environmental Factors:

Temperature and Light: Germination is often influenced by temperature and light conditions. Some seeds require a period of cold stratification or exposure to light to trigger germination.

Seed Dormancy: Certain seeds exhibit dormancy, a period of growth inactivity that allows them to survive adverse conditions. Breaking dormancy may require scarification (mechanical abrasion of the seed coat), exposure to smoke compounds, or specific temperature fluctuations.

Hormones: Hormones like gibberellins play a crucial role in regulating seed germination. They stimulate the growth of the embryo and mobilize stored energy reserves in the endosperm.

In conclusion, plant reproduction is a complex and diverse process involving both asexual and sexual mechanisms. It ensures the continuation of plant species and is influenced by various environmental factors and hormonal controls. Understanding these processes is vital for plant propagation and the cultivation of crops and ornamental plants.