Plant Anatomy: Secondary Growth, Xylem Transport, and Drought Adaptations
Xylem Pit Functionality in Angiosperms and Gymnosperms
Pits regulate the flow of water and solutes within the xylem. They are formed by the deposition of the endoplasmic reticulum on the primary wall, which prevents the formation of the secondary wall in these areas. When the organelle disappears, a thin cavity of primary wall remains. Subsequently, the primary wall is hydrolyzed, leaving a thin network of cellulose through which water and solutes can pass.
Comparison of Pit Structures
- Gymnosperms: Pits are areolate (bordered). They feature a torus (a greater development of the primary wall) connected to the margo (a thin cellulose section). In case of cavitation, the torus moves to one end, sealing the cell. Therefore, gymnosperms have a higher resistance to cavitation but a lower conduction rate.
- Angiosperms: Pits are simple. They lack a torus, possessing only a very thin membrane. In case of cavitation, the element is exposed to damage. Therefore, angiosperms have a lower resistance to cavitation but a higher conduction rate.
Development of Lateral Branches in Woody Plants
Branches of the primary stem originate from the eccentric vascular bundle, known as the leaf trace, which communicates with the primary vascular system. When secondary growth occurs, this foliar bundle remains eccentric and generates an area where no vascular cambium develops. This area, often called a leaf gap, is filled with parenchyma whose cells appear more or less organized. The leaf trace always communicates with the secondary vascular elements in the center of the stem.
In some cases, axillary buds appear, located above the leaf trace, possessing their own vascular bundle cords. When the bud is activated, a new pathway or a new branch is generated. Therefore, lateral branches proceed from the secondary xylem, crossing the cortex toward the exterior.
Formation and Maturation of Secondary Xylem Elements
The secondary plant body originates from lateral meristems, which derive from initial meristems. After the formation of the primary structure, two types of lateral meristems appear: the vascular cambium and the phellogen (suberous cambium). The vascular cambium forms the secondary conductive tissues.
Cell Division in Vascular Cambium
The division of cambial cells occurs in two ways:
- Tangential or Additive (Periclinal): Division occurs parallel to the surface. Cells formed toward the interior become xylem mother cells, and cells formed toward the exterior become phloem mother cells.
- Radial or Multiplicative (Anticlinal): Division occurs perpendicular to the surface, producing an increase in circumference with different patterns. In fusiform initials, division can be pseudo-transverse or radial; in ray initials, division occurs radially or by fragmentation.
Phases of Tracheary Element Maturation
The maturation process for a secondary xylem tracheary element (tracheid or vessel element) involves several critical phases:
- Expansion of the primary cell wall by increasing the size of the vacuole.
- Formation of the secondary wall in the xylem with deposition of cellulose and lignin.
- Degradation of the nucleus and organelles, followed by the rupture of the tonoplast (vacuole membrane). This lysis is often aided by substances released from the vacuole interior.
- After lysis, the protoplast is lost, and the contents are emptied through perforations (in vessel elements) or pits.
- The element is now ready to conduct water.
Leaf Adaptations to Drought: Nerium oleander (C3) vs. Cynodon dactylon (C4)
The leaf is a fundamental organ performing photosynthesis, gas exchange (CO2 assimilation, respiration, and transpiration), and transport of photoassimilates. Leaf form and structure are highly variable and related to the habitat and function. Adaptations to xeric (dry) environments are categorized as follows:
Types of Xeric Adaptations
- Morphological: Shape of leaves, orientation, vestiture (trichomes), and position of stomata.
- Anatomical: Thick cuticles, multistratified epidermis, and bulliform cells. These adaptations aim to minimize evapotranspiration by waterproofing the surface and creating an environment with higher relative humidity.
- Physiological: C4 metabolism and CAM metabolism. These adaptations aim to avoid photorespiration by separating the light and dark phases of photosynthesis spatially or temporally (separating RuBisCo and CO2 fixation).
Comparison Table
| Feature | Nerium oleander (C3) | Cynodon dactylon (C4) |
|---|---|---|
| Stomata Shape | Kidney shape | Elongated shape |
| Stomata Position | Located in a crypt (sunken) | Sunken into a cavity |
| Metabolism | C3 metabolism; light and dark phases occur without spatial separation. | C4 metabolism; light and dark phases are spatially separated. |
| Anatomy | Normal bifacial leaf anatomy:
| Kranz Anatomy: Mesophyll cells are arranged radially around the bundle sheath, which in turn surrounds the vascular bundles. Spongy parenchyma appears where the light phase and malate-shaped fixation occur. |
Periderm: Definition, Formation, Composition, and Functions
The secondary plant body originates from lateral meristems, derived from initial meristems. After the formation of the primary structure, two types of lateral meristems appear: the vascular cambium and the phellogen (suberous cambium).
Formation and Composition
The phellogen is generated from the epidermis, sub-epidermal cells, pericycle, or protophloem. It originates three layers:
- Toward the Outside: Phellem (Cork): Formed by periclinal divisions; a group of suberized cells.
- The Meristem: Phellogen (Cork Cambium).
- Toward the Interior: Phelloderm: A layer of cells often resembling parenchyma, sometimes with numerous intercellular spaces.
The periderm is a protective tissue that replaces the epidermis during secondary growth. It is formed by the set of phellem, phellogen, and phelloderm. As it forms, it expels the endodermis, cortical parenchyma, exodermis, and epidermis to the outside. Remnants of these tissues, along with the periderm and phloem, form the rhytidome (outer bark).
Function
The primary function of the periderm is to protect the plant from the breakage of primary tissues as the stem grows in thickness (widening the secondary xylem). It can also be generated in response to trauma or injury.
Phloem Structure and Function in Angiosperms and Gymnosperms
Phloem is a complex conductive tissue with diverse cell types. Its function is to transport photoassimilates (products of photosynthesis) throughout the plant.
Phloem Cell Types and Functionality
| Cell Type (Angiosperms) | Cell Type (Gymnosperms) | Function |
|---|---|---|
| Sieve Tube Elements | Sieve Cells | Conduction of sap |
| Companion Cells | Albuminous Cells | Regulation of assimilate flow and maintenance of sieve element viability |
| Axial Parenchyma | Axial Parenchyma | Storage and secretion |
| Fibers | Fibers (e.g., in cedar) | Protection (mechanical support) of phloem |
| Radial Parenchyma | Radial Parenchyma | Storage and secretion |
Functional and Adaptive Consequences
The primary phloem (protophloem and metaphloem) derives from the procambium and is associated with the xylem, forming conductive bundles (stem) or alternate poles (root); it does not contain radial parenchyma. The secondary phloem comes from the vascular cambium and forms a continuous ring-shaped tissue.
Angiosperm Specifics
In most dicotyledonous species, sieve tube elements are functional only during one vegetative season, being replaced by new ones. As the sieve element ages, the amount of callose increases, which is more abundant in angiosperms than in gymnosperms. In autumn, in some species, the sieve areas are blocked by callose, stopping the flow of sap. In spring, the cytoplasm dissolves the callose by enzymatic action, and sap passage is renewed.
P-protein (Phloem Protein) is very important in dicotyledonous sieve tubes, rare in monocots, and absent in gymnosperms. If the cell is damaged, P-protein disperses throughout the cellular lumen and obstructs the pores of the sieve areas, stopping the exudation of sap. It also synthesizes callose rapidly in response to injury.