Simplified Plant TissuesLesson Plan 4

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Plant Organs

The 3 primary organs found in plants are:• Leaves• Stems• Roots

Since flowers are thought to have evolved from leaves, they are called “reproductive structures”.

Leaves, stems, roots, and flowers are all made up of various tissues.

Plant Tissues

Plant Tissues fall into two large categories:

• Meristematic, or

• Permanent (Non-Meristematic)

Meristematic Tissues

Meristems are places in plants where cell division is occurring.

During cell division, one cell becomes two cells. Each new cell can also divide.

Meristematic Tissues

In this class we will discuss three kinds of meristems:

• Apical Meristems• Intercalary Meristems• Lateral Meristems

Apical Meristems In plants belonging to

the DICOT class, apical meristems are located in BOTH the shoot tips and root tips. A shoot is simply a young, leaf-bearing stem.

In plants belonging to the MONOCOT class, apical meristems are located ONLY in the root tips.

Microscopic View of an Apical Meristem

Apical Meristems

Apical meristems cause PRIMARY GROWTH.

As cells in apical meristems divide and elongate, shoot tips and root tips grow longer. This increase in length is called primary growth.

Apical Meristem in a Root Tip

Apical Meristems•Primary growth of shoots and roots occurs as the cells of the apical meristem divide and elongate.

Apical Meristems

A bud is a dormant apical meristem.

When conditions are favorable for growth, buds “break” and begin pushing out new growth.

Bud Break

Buds

Buds come in 3 basic types:• Vegetative-produce stems and leaves.• Flower-produce only flowers.• Mixed-produce stems/leaves and flowers.

Mixed Buds

Mixed buds of Amelanchier x grandiflora

Mixed Buds of Cornus alternifolia

1 2

3 4

Pussy WillowPussy Willow is a plant which bears separate vegetative and flower buds. In the image to the left you can see vegetative buds producing green leaves and flower buds producing male flowers (the species is dioecious). Also, note that the two different types of buds are indistinguishable from one another.

vegetative bud

flower bud

Lantanaphyllum Viburnum

Viburnum x rhytidophylloides, Lantanaphyllum Viburnum, also bears separate vegetative and flower buds. However, the two types of buds are distinctly different from one another in appearance.

flower bud vegetative bud

Intercalary Meristems

In addition to the apical meristems located in their root tips, plants in the MONOCOT class have special meristems called intercalary meristems.

Intercalary meristems are located on monocots where the leaf bases connect to the stem of the plant.

Intercalary Meristems

Monocots, like grasses, have intercalary meristems which allow the leaves to grow

back after mowing.

Lateral Meristems In addition to the

apical meristems located in the shoot and root tips, plants in the DICOT class have lateral meristems.

Lateral meristems cause SECONDARY GROWTH.

Secondary growth causes stems and roots to grow larger in diameter.

Lateral Meristems cause an increase in stem diameter

Lateral Meristems

Think of a lateral meristem as being a cylinder, several cell layers thick, of actively dividing cells.

Lateral Meristems MONOCOTS do not have lateral

meristems. Lateral meristems come in two basic types: Vascular cambium; meristematic tissue

located between the xylem and phloem. The vascular cambium makes new xylem and phloem cells.

Cork cambium; meristematic tissue located just below the bark in woody dicot stems. The cork cambium makes new bark cells.

Lateral Meristems

Phloem

Vascular Cambium

Xylem (Wood)Cross Section of a Woody Dicot Stem

Lateral Meristems

Cork (Bark)

Cork Cambium

Vascular Cambium

Woody Dicot Stem

Permanent Tissues Permanent tissues are non-meristematic. The following permanent tissues will be

discussed:• epidermis

• xylem

• phloem

• cortex

• pith

• fibers

Epidermis

The epidermis can be from one to several cell layers thick.

It covers all the parts of a plant; shoots, stems, leaves, and roots.

In woody dicots the epidermis is called the bark or cork.

Agave Epidermis

Epidermis The epidermis

covering the leaves of plants contains thousands of tiny pores called stomata. The singular of stomata is stoma.

Stomata are usually found on the underside of leaves.

An apple leaf contains 39,000 stomata per square centimeter!

Stoma in Fuchsia

Epidermis

In most plants the cells of the upper and lower leaf epidermis secrete a waxy substance that covers the leaf and prevents water loss.

This waxy secretion is called a cuticle. Leaf with Cuticle

Cuticle

Epidermis

In many plants, the cells of the upper and lower leaf epidermis are modified to form small hairs.

These small hairs are called pubescence.

Lambs Ear is an example of a herbaceous perennial that has pubescent leaves.

Leaf Hair

Cortex

The cortex is a tissue found in both the stems and roots of monocots and dicots.

The cortex is made up of thin-walled cells. Vascular bundles are surrounded by the

cortex.

Cortex

Dicot Stem Monocot Stem

Vascular Bundles

Dicots-Cortex and Pith

Vascular Bundle

Cortex

In this dicot stem cross section the cortex, vascular bundles, and pith have been identified.

The pith is located in the center of dicot stems.

Pith

Monocots-Cortex

In this monocot stem cross section the cortex and vascular bundles have been identified.

Monocots do not have a pith.

Cortex Vascular Bundle

Xylem Xylem tissue conducts

water and dissolved mineral nutrients.

It is composed of columns of vessel cells (tracheids in gymnosperms), stacked one upon another, and fibers.

Vessel cell walls are pitted and allow water passage between the xylem and surrounding tissues. Mature xylem cells are dead.Xylem Phloem

fibers not shown

Phloem

Phloem tissue conducts the food produced by the leaves in a downwards direction to stems and roots (from source to sink).

Phloem is composed of sieve tubes, companion cells, and fibers.

The end walls of the sieve tubes are called sieve plates.

Xylem Phloem

fibers not shown

Fibers

Fiber cells have thick walls. A cluster of fiber cells is called a fiber.

Fibers are associated with the xylem and phloem of both monocots and dicots.

Fibers provide strength and support to xylem and phloem.

Fiber Cell - the cell wall is stained dark red.

Fibers

A cluster of many fiber cells make up a fiber

Fibers

Cortex

Xylem

Phloem

Fibers

Vascular Bundles in Corn

The End