Plants exhibit a large size with a branching shape which is advantageous for maintaining a large Surface Area to Volume (SA:V) ratio.
The leaves of plants are characterized by thinness.
Plants possess two primary transport systems:
Xylem: Responsible for the transport of water and inorganic ions from the roots to the leaves.
Phloem: Facilitates the transport of organic substances produced through photosynthesis from the leaves to other parts of the plant.
01.1 Transverse Section of Roots, Stems, and Leaves of Herbaceous Dicotyledonous Plants
Roots, stems, and leaves are crucial organs for transport within plants.
Flowering plants are classified as monocotyledons or dicotyledons, each with distinct characteristics.
While the transport mechanisms in both types are similar, there are notable differences in the distribution of xylem and phloem among the roots, stems, and leaves of monocots and dicots.
For example:
Monocotyledons (e.g., grasses) typically display long, narrow leaves.
Dicotyledons feature leaves with distinct blades and stalks.
02. Transverse Sections of Various Plant Organs
Root Structure:
Composed of epidermis, cortex (made of parenchyma cells), and vascular tissues (xylem and phloem).
Contains a pericycle and endodermis.
Stem Structure:
Epidermis: Outermost protective layer.
Cortex: Composed of collenchyma for support and parenchyma for storage.
Vascular Bundle: Contains both xylem and phloem surrounded by bundle sheath fibers.
Leaf Structure:
Upper Epidermis: A thin, transparent layer that allows light penetration; covered with cuticle to reduce water loss.
Palisade Mesophyll: Parenchyma cells with chloroplasts, primarily for photosynthesis.
Lower Epidermis: Contains stomata for gas exchange.
Spongy Mesophyll: Features large air spaces for carbon dioxide circulation and chloroplasts for photosynthesis.
03. Detailed Cell Types
Epidermis Cells:
A continuous, one-cell-thick layer providing external protection.
Covered in a waterproof cuticle, especially in stems and leaves, to prevent desiccation and infection.
In roots, root hairs extend to enhance absorption efficiency.
Parenchyma Cells:
Contain chloroplasts in leaves forming the palisade and spongy mesophyll.
Form the cortex and pith in roots and stems.
Functions include:
Serving as packing tissue.
Active metabolism and food storage (e.g., starch).
Providing turgor for structural support.
Facilitating gas exchange via intercellular air spaces.
Collenchyma Cells:
Modified parenchyma with added cellulose for enhanced strength, especially found in the midrib of leaves.
Endodermis:
A single cell layer encircling vascular tissues in stems and roots.
Mesophyll:
Specialized parenchyma between upper and lower epidermis involved in photosynthesis, containing chloroplasts.
Comprises palisade (upper) and spongy mesophyll (lower).
Pericycle:
A layer adjacent to vascular tissues in roots and stems:
In roots, it's one cell thick, allowing for new root formation.
In stems, it is sclerenchyma tissue providing strength.
04. Vascular Tissues (Xylem and Phloem)
Comprise the xylem and phloem, arranged in vascular bundles in stems:
Xylem Tissue:
Develops from vessel elements that stack end-to-end, transform into tubes through lignification (cell dies and forms a lumen).
Characteristics include:
Joined end-to-end to create continuous tubes.
Consists of dead cells with thick lignified walls.
Has pits for water movement between vessel elements.
Functions include providing structural support and transporting water and minerals.
Phloem:
Comprises living sieve tubes that transport assimilates. (Further explained in a subsequent section.)
05. Water and Mineral Movement in Xylem
Movement of water is passive and primarily driven by evaporation from leaves, following the path: soil → roots → stems → leaves.
Stages of water transport include:
From leaves into the atmosphere, where moisture evaporates from spongy mesophyll cells through stomata.
In the stem, drawing water through xylem to replenish lost amounts post-evaporation.
From root hairs into xylem.
From soil into root hairs, facilitated by osmosis and the high surface area of root hairs.
06. Factors Affecting Water Movement
Water vapor escapes through stomata, leading to transpiration, characterized by:
Increased transpiration rate with higher temperatures.
Decreased transpiration with higher humidity.
Increased transpiration rate with higher wind speeds.
Increased transpiration due to high light intensity as stomata remain open for photosynthesis.
07. Xerophytes and Adaptations
Xerophytes are plants adapted to arid environments with special leaf adaptations to minimize water loss:
Ability to roll leaves, exposing only a thick cuticle.
Stomata location only on upper epidermis, reducing exposure to external air.
Presence of hairs trapping moisture close to the leaf surface.
Smaller leaf surface area to volume ratio to reduce water vapor loss.
08. Transport of Assimilates in Phloem (Translocation)
Movement of substances like sucrose and amino acids through phloem, termed translocation:
Source: Location of assimilate production (e.g., leaves).
Sink: Location of assimilate consumption or storage.
Phloem contains:
Sieve Tube Elements: Living cells forming the tubes, lacking nuclei but containing cytoplasm and endoplasmic reticulum.
Companion Cells: Associated with sieve elements, crucial for metabolic support, contain a nucleus, and are rich in ribosomes.
09. Mechanism of Translocation
09.(A) Loading of Sucrose into Phloem
Conversion of glucose (from photosynthesis) into sucrose.
Proton pump creates a concentration gradient, enabling H+ ions to re-enter companion cells through co-transporter proteins, moving sucrose into sieve tubes.
09.(B) Unloading of Sucrose from Phloem
(Near source:) Increased sucrose concentration lowers water potential, causing osmosis from xylem into sieve elements, generating hydrostatic pressure.
(At sink:) Sucrose diffuses out of the phloem, maintaining living cell function, resulting in osmotic water movement back towards xylem, creating pressure flow throughout phloem.