Unit 7 Plants

Ch. 23 Roots, Stems, & Leaves

Seed Plant Structure

• 3 of the principal organs of seed plants are roots, stems, & leaves

• The root system of a plant absorbs water & dissolved nutrients

Seed Plant Structure

- A stem has a support system for the plant body, a transport system that carries nutrients, & a defense system

that protects the plant against predators & disease

- Leaves are the plant’s main photosynthetic systems

Plant Tissue Systems

• Plants consist of 3 main tissue systems: dermal tissue, vascular tissue, & ground tissue

Dermal Tissue

• The outer covering of a plant, that consists of a single layer of epidermal cells

• The outer surfaces are often covered with thick waxy layer to protect against water loss & injury

Vascular Tissue

• Contains several types of specialized cells

• Xylem consists of tracheids & vessel elements

• Phloem consists of sieve tube elements & companion cells

Vascular Tissue - Xylem

• All seed plants have a type of xylem cell called a tracheid, cells that resist water pressure

• Vessel elements - wide cells that form hollow tubes, which water can move through freely

Vascular Tissue - Phloem

• Sieve tube elements - main cells in phloem, form tubes through which materials, like sugars & other foods, are carried in a watery stream

• Companion cells - phloem cells that surround sieve tube elements, help movement of substances in & out of the phloem

Ground Tissue

• Cells that lie between dermal & vascular tissues

• Consists mainly of parenchyma - cells that are packed with chloroplasts & are the site of most of a plant’s photosynthesis

Ground Tissue

• Collenchyma - cells that have strong, flexible cell walls that help support larger plants (makes up “strings” of a stalk of celery)

• Sclerenchyma - rigid cell walls that make ground tissue tough & strong

Plant Growth & Meristematic Tissue

• Meristems - clusters of tissue that are responsible for continuing growth throughout a plant’s lifetime

• Meristematic tissue - undifferentiated cells, they have not yet become specialized for specific functions, such as transport

Plant Growth & Meristematic Tissue

• Apical meristem - group of undifferentiated cells that divide to produce increased length of stems & roots

Plant Growth & Meristematic Tissue

• Differentiation - cells develop into mature cells with specialized structures & functions

• As cells differentiate, they produce each of the tissue systems of the plant: dermal, ground, & vascular tissue

Types of Roots

• The 2 main types of roots are taproots, found mainly in dicots, & fibrous roots, found mainly in monocots

• Taproot - primary root– Ex.) carrots, dandelions, beets, etc.

Types of Roots

• Fibrous roots - roots that branch to such an extent that no single root grows larger than the rest– Ex.) grass

Root Structure & Growth

• A mature root has an outsider layer, the epidermis, & a central cylinder of vascular tissue

• Between these 2 tissues, lies a large area of ground tissue

Root Structure & Growth

• Root hairs - tiny cellular projections that penetrate the spaces between soil particles & produce a large surface area through which water can enter the plant

Root Structure & Growth

• Cortex - spongy layer of ground tissue just inside the epidermis

• Endodermis - another layer of cells that completely encloses the root’s vascular subsystem in a region called the vascular cylinder

Root Structure & Growth

• Root cap - protects the root as it forces its way through the soil

Root Functions

• Roots anchor a plant in the ground & absorb water & dissolved nutrients from the soil

Stem Structure & Function

• Stems have 3 important functions: they produce leaves, branches, & flowers; they hold leaves up to the sunlight; & they transport substances between roots & leaves

Stem Structure & Function

• Nodes - where leaves are attached

• Internode - regions between the nodes

• Buds - contain undeveloped tissue that can produce new stems & leaves

Monocot & Dicot Stems

• In monocots, vascular bundles are scattered throughout the stem

• In dicots & most gymnosperms, vascular bundles are arranged in a cylinder

Monocot & Dicot Stems

• Vascular bundles - contains xylem & phloem tissue

• Pith - the parenchyma cells inside the ring of vascular tissue

Primary Growth of Stems

• Primary growth - growth occurring only at the ends of a plant

• Its produced by cell divisions in the apical meristem, & takes place in all seed plants

Secondary Growth of Stems

• Secondary growth - method of growth where stems increase in width

• In conifers & dicots, secondary growth takes place in lateral meristematic tissues called the vascular cambium & cork cambium

Secondary Growth of Stems

• Vascular cambium - produces vascular tissues & increases the thickness of stems over time

• Cork cambium - produces the outer covering of stems

Formation of Wood

• Heartwood - older xylem near the center of the stem that no longer conducts water

• Sapwood - surrounds heartwood, active in fluid transport

Formation of Bark

• Bark - includes all of the tissues outside the vascular cambium, includes: phloem, the cork cambium, & cork

Leaf Structure

• The structure of a leaf is optimized for absorbing light & carrying out photosynthesis

Leaf Structure

• Blades - flattened section, attached to the stem by a thin stalk - petiole

Leaf Functions

• A leaf can be considered a system specialized for photosynthesis

• Subsystems of the leaf include tissues that bring gases, water, & nutrients to the cells that carry out photosynthesis

Leaf Functions

• Guard cells - specialized cells in the epidermis that control the opening & closing of stomata (air spaces) by responding to changes in water pressure

Leaf Functions

• Plants keep their stomata open just enough to allow photosynthesis to take place, but not so much that they lose an excessive amount of water

Leaf Functions

• Transpiration - the loss of water through its leaves