Life Cycle Length of Flowering Plants

Life Cycle Length

• Flowering plants are classified by life cycle length:
  • Annuals complete life cycle in one year or less.
  • Biennials require two growing seasons.
  • Perennials live for multiple years.

Primary Growth

A. Root Growth

• Root tip is protected by a root cap, enabling soil penetration.
• Growth occurs in three zones:
  • Zone of cell division
  • Zone of elongation
  • Zone of differentiation
• Primary roots produce epidermis, ground tissue, and vascular tissue.
• In angiosperms, the stele forms a vascular cylinder.
• Eudicot xylem has a starlike arrangement with phloem between the arms.
• Monocots exhibit a core of parenchyma surrounded by rings of xylem and phloem.
• Ground tissue (mostly parenchyma) fills the cortex, between vascular cylinder and epidermis.
• The endodermis regulates nutrient passage to vascular cylinder.
• Lateral roots form from the pericycle, the outermost layer in vascular cylinder.

B. Shoot Growth

• The shoot apical meristem is located at the shoot tip, consisting of dividing cells.
• Leaves develop from leaf primordia on the apical meristem sides.
• Axillary buds arise from meristematic cells at leaf primordial bases.

C. Tissue Organization of Stems

• Lateral shoots grow from axillary buds on stem surface.
• Eudicot vascular tissue forms as bundles arranged in a ring.
• In monocots, vascular bundles are scattered throughout the ground tissue.

D. Tissue Organization of Leaves

• The leaf epidermis has stomata for gas exchange (CO2 & O2).
• Stomatal pores are flanked by guard cells that control their opening/closing.
• Ground tissue in leaves, called mesophyll, is between upper and lower epidermis.
• Eudicot mesophyll has two layers:
  • Palisade mesophyll (upper layer)
  • Spongy mesophyll (lower layer) allows gas exchange.
• Leaf vascular tissue connects to the stem's vascular system.
• Leaf veins serve as vascular bundles and structural support.
• Each vein is enclosed by a protective bundle sheath.

Secondary Growth

A. General Overview

• Secondary growth occurs in the stems and roots of woody plants but is rare in leaves.
• The secondary plant body consists of tissues from vascular cambium and cork cambium.
• Characteristic of gymnosperms and many eudicots, less common in monocots.

B. Vascular Cambium

• Vascular cambium is a one-cell-layer thick meristem located as a cylinder.
• Arises from undifferentiated parenchyma cells.
• Appears as a ring of initials in cross-section.
• Increases circumference, adds secondary xylem internally and secondary phloem externally.
• Secondary xylem accumulates as wood, consisting of tracheids and vessel elements (only in angiosperms).
• Early wood has thin walls for efficient water delivery; late wood has thick walls contributing to support.
• Vascular cambium is inactive in temperate perennials during winter.
• Eudicots and gymnosperms typically feature a taproot system comprising a main taproot and lateral roots.
• Monocots utilize a fibrous root system with adventitious roots and lateral roots from them.

C. Stems

• Stems alternate nodes (leaf attachment points) and internodes (segments between nodes).
• An axillary bud can develop into a lateral shoot, while an apical (terminal) bud drives elongation at shoot tip.
• Apical dominance keeps most axillary buds dormant.
• Many plants have modified stems, such as rhizomes, bulbs, stolons, and tubers.

D. Leaves

• Leaves are primary photosynthetic organs of vascular plants.
• Structure includes a flattened blade and a stalk (petiole) connecting the leaf to stem nodes.
• Monocots have parallel vein arrangements; eudicots exhibit branching vein patterns.
• Leaf morphology can be a criterion for classifying angiosperms.

E. Tissue Types

1. Dermal Tissues
   • Nonwoody plants feature epidermis with a waxy cuticle to reduce water loss.
   • In woody plants, periderm replaces epidermis in older stems and roots.
   • Trichomes (epidermal outgrowths) can defend against insects.

Plant Growth Overview

• Plants demonstrate indeterminate growth, allowing continuous growth throughout their lives in optimal conditions.
• In contrast, animals exhibit determinate growth, where size is limited by genetic factors.
• Key organs such as leaves, flowers, and fruits reach a maximum size and do not grow further.

Growth Mechanisms

• Growth in plants is driven by meristems, specialized cells located in distinct areas:
  • Apical Meristems are located at shoot and root tips, facilitating primary growth which increases height.
• Lateral Meristems include:
  • Vascular Cambium: Responsible for adding new layers of vascular tissues, namely xylem and phloem.
  • Cork Cambium: Produces a protective layer called periderm, replacing the outer epidermis.

Types of Growth

• Primary Growth occurs at the tips of stems and roots, directing growth towards sunlight and water sources and involves apical meristems.
• Secondary Growth involves lateral meristems and contributes to the increase in a plant's thickness, enhancing structural support through new xylem and phloem layers.

Plant Life Cycle

• Annuals complete their lifecycle within a single year.
• Biennials require two years to finish their lifecycle, often with a growth cycle in each year.
• Perennials live for multiple years, completing their lifecycle over an extended period.

Plant Anatomy Insights

• Nodes are the points on stems where new shoots and branches develop.
• Bud scars mark previous growth periods on a stem.
• Roots are organized into distinct zones:
  • Zone of Cell Division: Contains apical meristem for cell production.
  • Zone of Elongation: Responsible for the lengthening of roots through cell elongation.
  • Zone of Differentiation: Involves specialization of cells for various functions such as nutrient storage and protective roles.

Root Structure and Function

• Root Cap: Protects the root tip during soil penetration, shielding delicate meristematic tissue.
• Cortex: Made from parenchyma cells, this tissue facilitates storage and transport within the root.
• Vascular Cylinder: Houses xylem and phloem, crucial for transporting water, minerals, and nutrients throughout the plant.
• Endodermis: Acts as a selective barrier that regulates the entry of substances into the vascular system.

Summary of Tissue Types

• Parenchyma Cells are involved in storage, such as starch, and serve vital metabolic functions within the cortex.
• Xylem is specialized for water and mineral transport and consists of larger vessel elements and smaller tracheids.
• Phloem transports organic nutrients like sugars and contains sieve tube elements, vital for nutrient distribution.

Key Functional Roles

• The endodermis prevents the entry of potentially harmful materials into the xylem.
• Lateral roots originate from the pericycle, integrating into vascular transport systems for efficient nutrient and water distribution.

Secondary Growth in Plants

• In perennial plants, secondary growth enlarges the stems, trunks, or branches.
• Annual plants and most monocots do not exhibit secondary growth.

Meristematic Tissues

• Secondary growth is driven by two types of meristems: vascular cambium and cork cambium.
• Vascular cambium produces secondary xylem (wood) and secondary phloem (bark).
• Cork cambium forms the periderm, the protective outer bark layer.

Vascular Cambium Function

• Vascular cambium is a layer of cells located along the length of stems and trunks.
• Cells in the vascular cambium can either replicate or differentiate into secondary xylem and phloem.
• Accumulation of secondary xylem over years creates heartwood, while secondary phloem is pushed outward.

Growth Rings

• Tree growth rings indicate annual production of secondary xylem and phloem.
• Early wood is produced in spring, characterized by larger vessel elements due to high moisture availability.
• Late wood forms under drier conditions, featuring smaller vessels.

Heartwood and Sapwood

• Heartwood consists of older, non-functioning secondary xylem, often darker in appearance.
• Sapwood is the outer layer of functional xylem that actively conducts water.

Cork Cambium and Protective Structures

• Cork cambium produces cork cells that create a waterproof protective layer due to the presence of suberin.
• Bark includes both cork and the outer layers of secondary phloem.

Gas Exchange

• Lenticels are small pores in the bark that enable gas exchange, allowing oxygen to reach living tissues in the secondary phloem.

Overview of Bark

• Bark comprises the periderm (made of cork cambium and cork) along with all secondary phloem, differing from common usage that often refers only to the cork.

Plant Tissue Types

• Three primary categories of plant tissues: dermal, ground, and vascular.
• Dermal tissue: Protects the plant; includes the epidermis.
• Ground tissue: Involved in metabolic functions and storage; includes parenchyma, collenchyma, and sclerenchyma.
• Vascular tissue: Facilitates the transport of water, nutrients, and sugars.

Leaf Structure

• Leaves function as organs due to their composition of various tissue types.
• Eudicot leaves feature distinct anatomical traits that aid in their functions.

Epidermis

• The upper and lower epidermis serve as protective layers for the leaf.
• The epidermis secretes a waxy cuticle, which minimizes water loss from the leaf surface.
• Contains stomata, small openings that enable the regulation of gas exchange.

Stomata and Guard Cells

• Stomata function like human nostrils, allowing for air exchange essential for photosynthesis.
• Guard cells regulate the opening and closing of stomatal pores by swelling or shrinking in response to water levels.

Mesophyll Types

• Mesophyll is the tissue responsible for photosynthesis, consisting of specialized parenchyma cells.
• Palisade mesophyll: Comprises elongated, columnar cells situated near the upper epidermis; maximizes light absorption.
• Spongy mesophyll: Contains irregularly shaped cells located near the lower epidermis, featuring air spaces that facilitate gas exchange.

Gas Exchange and Photosynthesis

• Carbon dioxide enters the leaf through stomata, while oxygen, a byproduct of photosynthesis, is released from the same openings.
• Air spaces within spongy mesophyll enhance the circulation and exchange of gases.

Vascular Structure

• Leaf veins are composed of vascular bundles that contain both xylem and phloem tissues.
• Vascular bundles are encased in bundle sheath cells for added protection.
• Sclerenchyma fibers contribute structural support to prevent damage and maintain the integrity of the vascular system.