12 Secondary growth
Secondary Growth
Formation
Secondary growth requires the formation of a secondary vascular system.
This system consists of:
Secondary xylem
Secondary phloem
Originates from the vascular cambium.
Additional requirements for secondary growth include the development of periderm.
Periderm primarily comprises cork produced by the cork cambium.
Notably, secondary growth rarely occurs in monocots and only minimally in herbaceous eudicots, which generally rely on the primary vascular system.
Insight: If xylem represents highways, then secondary xylem and phloem function as rural roads in plant transport systems.
Formation of the Secondary Components
Vascular Cambium Development
The vascular cambium is generated from meristematic divisions within the procambium in non-elongating regions.
The cambium cells are positioned between primary xylem and phloem.
Cellular division begins in regions internal to the phloem strands, forming initial cambium primordia.
Pericycle cells located opposite the primary xylem also undergo division.
The inner sister cells transition into vascular cambium, encircling the xylem by meeting with other cambium primordia.
Secondary Xylem and Phloem Production
The vascular cambium initiates division towards the interior, leading to the formation of secondary xylem.
This division causes the primary phloem to be pushed towards the periphery, effectively separating it from the primary xylem.
Division processes at both the interior and exterior of the vascular cambium continue until the development of secondary xylem and phloem is fully accomplished.
Parenchyma cells form rays extending throughout the secondary xylem and phloem.
As the width of secondary tissues increases, the primary phloem becomes crushed, making it increasingly difficult to visually distinguish.
Rays: Act as conduits for movement between xylem cells, with xylem being composed of dead tissue, while rays remain alive for the transport of sugars and other nutrients.
Structure of Woody Roots
In woody roots, a layer of periderm replaces the outer epidermis, leading to significant structural modifications.
The periderm is formed from the pericycle and only emerges following the initiation of the secondary vascular system.
As roots widen, the epidermis and cortex face rupturing due to internal pressure.
The cork cambium produces cork cells to the exterior while generating thin-walled phelloderm cells towards the inner side.
Cork functions as a protective, dead tissue layer containing suberin and forms a waterproof barrier.
Phelloderm remains alive and serves various functions in protection and repair.
Other remaining pericycle cells produce new tissues akin to the shed cortex.
Critical Note: Periderm does not form if the secondary vascular system is not established; consequently, live cortex cells die, increasing susceptibility to invasion and water loss.
Production of Lateral Roots
Lateral Roots arise from the pericycle, positioned opposite the protoxylem poles.
Characterized by endogenous origin, they emerge after the zones of elongation in partially or fully differentiated plant tissues.
As lateral root primordia commence growth, they pierce through the cortex, often aided by enzymes that degrade cortical cells.
Lateral roots swiftly develop a root cap to shield the apical meristem during growth.
The vascular system of lateral roots is initially independent of the parent root but eventually connects as the pericycle and vascular parenchyma establish their xylem and phloem connections.
Specialized Root Types
Aerial Roots
Roots that develop aboveground, serving various structural roles, including shoot support.
Initiate branching upon contacting the ground and commence absorption processes.
Prop roots are a common example found in maize, providing support, while stilt roots are prevalent in tropical plants.
Additional forms include roots from climbing species that do not penetrate soil but support the plant structurally.
Pneumatophores
A distinctive trait of certain plant species inhabiting waterlogged areas.
These roots exhibit negative gravitropism, growing upwards.
Their design allows roots to access air, essential for respiration as they require oxygen.
Epiphytes
These plants grow on other plants without being parasitic.
Adaptations include multi-layered thick root epidermis to mitigate water loss, with some species featuring photosynthetic capabilities.
The specialized epidermis, known as velamen, facilitates water and nutrient absorption and storage.
Fleshy, Storage Roots
While most roots function as storage organs, certain types become more specialized and fleshy.
Characterized by a significant presence of parenchyma intermixed with vascular tissues.
Development patterns are similar to other root types, except with abundant parenchyma surrounding secondary xylem and phloem.
Example: Carrot serves as a classic illustration, while sweet potatoes exhibit additional vascular cambium development around secondary xylem.
Increased cambia lead to extensive storage parenchyma production, while sugar beets demonstrate thickness mainly due to added cambia surrounding the original vascular cambium.
Mycorrhizal Symbionts of Roots
Mycorrhizal Associations
Two primary classes of mycorrhizal fungi:
Arbuscular Mycorrhizal Fungi (AM Fungi): Form arbuscules within root cortical cells and are crucial for nutrient exchange.
Ectomycorrhizal Fungi (EcM Fungi): Develop a Hartig net around the root, functioning as an extended root system.
AM fungi scavenge inorganic nutrients, while EcM fungi break down organic matter.
Arbuscular Mycorrhizal Fungi (AM Fungi)
Belong to the phylum Glomeromycota.
Establish symbiosis with around 80% of terrestrial plants, functioning as obligate biotrophs.
Plants secrete strigolactones to stimulate AM fungal hyphae directional growth.
Upon contact, AM fungi form hyphopodia on the root surface penetrating the epidermis and cortical layers without breaching the endodermis or vascular system.
Inside cortical cells, they create arbuscules, enhancing nutrient exchange efficiency.
Vesicles can also form to store lipids, aiding fungal persistence.
Ectomycorrhizal Fungi (EcM Fungi)
These fungi belong to the phyla Basidiomycota or Ascomycota.
Predominantly reproduce aboveground, while associating with roughly 2% of plant species.
They are essential in temperate and boreal forest ecosystems and behave as facultative heterotrophs.
Roots secrete auxin-like compounds to modulate immune responses and compatibility with the fungus.
The fungi envelop fine roots with dense hyphal sheaths, while forming the Hartig net, serving as the primary interface for nutrient exchanges, including breakdown of organic materials in the soil for plant consumption.