NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 29

PLANT TISSUES

The Angiosperms are divided 3 major groups:

                     Magnoliids

Monocots

Grasses, Grains, Lilies, and Palms

                     Dicots

Two of these groups (Monocots versus Dicots) show some basic design differences (See fig 29.10).

The basic design of land plants has two parts (See 29.2):

          -Root

          -Shoot

Plants have three main tissue systems (See fig. 29.2)

          -Dermal Tissue System – from the protoderm - outside covering

          -Ground Tissue System – from the ground meristem - the in-between stuff

          -Vascular Tissue System – from the procambium - the pipes

>Plant growth and development

Plants grow only in special regions. The cells in these regions are meristem cells.

Meristem cells are undifferentiated (totipotent) embryonic cells. This means the cells have not become specialized.

Once plant cells mature they become differentiated cells and usually don’t divide again.

Plant growth is mainly two-dimensional (up and down) and as a result plants grow longer not wider. This is the result of the location of the meristem cells.

Meristem cells are found at both ends of the plant. These cells at the root and shoot tips are the apical meristems (See figs 29.4)

The growth that occurs at the apical meristem is called primary growth. Primary growth continues throughout the life of a plant and is responsible for the increase in height of a plant.

The other location of meristem cells is the lateral meristem.

The lateral meristem is found in ring like structures, one called the vascular cambium and the other the cork cambium (See fig. 29.4).

The vascular cambium the xylem and phloem and is responsible for secondary growth.

Secondary growth occurs later in the life of a plant and is responsible for the thickening of branches and trunks.

The cork cambium is responsible epidermal replacement.

The ground tissue system is a simple tissue system and is composed of three tissue types (See fig 29.6):

          -Parenchyma tissue

          -Collenchyma tissue

          -Sclerenchyma tissue

The parenchyma tissue has thin-walled living cells.

In the shoots and leaves these are the main photosynthesizers. In roots they are the main storage tissue for the plant.

The collenchyma tissue has elongated living cells with irregular thick walls. These cells provide support and are tough but flexible. (e.g. celery stalk)

The sclerenchyma tissue is made up of dead cells with very thick walls that have added lignin. These cells provide strength and support and are very hard (e.g. pits, nut shells, etc.)

There are two types of complex tissues systems:

                     vascular tissue system

                     dermal tissue system

There are tissue types that make up the vascular tissue system

          -Xylem

          -Phloem

The xylem conducts water and minerals from the roots to the rest of the plant.

Two cell types are found in the xylem (See fig 29.8):

          -Tracheids

          -Vessel elements

Both of these cell types in the xylem are dead cells.

Tracheids are small diameter pipes with overlapping slanted ends. The ends of the tracheid cells have pits that allow water to pass from cell-to-cell.

The vessel elements are large diameter pipes. The vessel element cells meet end-to-end and the wall where the ends meet is either perforated or missing entirely.

The phloem conducts water, sugar, amino acids and hormones from some source to rest of the plant or to a “sink”.

Two cell types are found in the phloem (See fig. 29.8):

          -Sieve tube elements

          -Companion cells

Both of these cell types contain cells that are living.

The sieve tube elements are arranged similar to the xylem. The cells are end-to-end.

The cell wall where two cells meet has holes in it and forms a structure called a sieve plate.

The plasma membrane of each adjacent cell is fused to the openings of the sieve plate. So this pipe is lined with living tissue.

The cells of the sieve tube while they are alive have lost many of their cellular components (no nucleus, very few ribosomes, and very little endoplasmic reticulum).

Because these cells have lost so many of their important components they are maintained and controlled by companion cells.

Two tissue types make up the dermal tissue system.

          -Epidermal tissue

          -Periderm

The epidermal tissue forms the epidermis (skin) that covers the outside of the plant.

          -Made up of thin-walled cell with a waxy cuticle.

Some epidermal cells produce fine hair-like structures called root hairs or leaf hairs.

The periderm tissue arises from the cork cambium and replaces the epidermis in older woody stems, branches and trunks.

          -Made up of thick-walled cork cells

These cork cells are waterproof and help form the bark

STEMS AND SHOOTS

As the shoot of a plant grows at the apical meristem it gives rise to different specialized tissues:

          Stems

          Buds

          Leaves

          Flowers

All of shoot structures come from small groups of cells which are left behind by the apical meristem.

These groups of cells form leaf primordia (immature leaves) and lateral buds (branch producing group).

The inside of the dicot stem contains the ring of vascular bundles and ground tissue.

The ground tissue that is inside the ring is known as pith whereas the ground tissue outside the ring is the cortex (See fig. 29.13)

LEAVES

Leaves of a plant have two main parts the blade and the petiole (the leaf stem) (See fig 29.14)

The structure of the leaf is designed in layers (See fig. 29.16).

The two outer layers (top and bottom) are epidermis and in the epidermal layer on the underside of the leaf there an opening called stomata.

Between these epidermal layers is the mesophyll (middle).

The cells here are parenchyma cells and they are arranged in a row of column shaped palisade cells toward the topside of the leaf.

The bottom side of the mesophyll is spongy cells with many air spaces.

THE ROOT SYSTEM

As a seed begins to grow the first root to emerge is the primary root.

In dicots the primary roots turn into the taproot system, which has one main root that all the other roots branch off.

In monocots this primary root is replaced by a fibrous root system composed of many roots of about equal size.

Roots grow by primary growth and the apical meristem cells are located underneath a layer of cell called the root cap (See fig. 29.17).

This root cap acts like a lubricating layer as the root pushes its way down through the soil. The cells of the root cap secrete a slippery slime layer and regularly slough off to make penetration easier.

The epidermis of the root is very thin and has no waxy cuticle. In addition, the epidermis forms root hairs to increase the surface area. These characteristics make the root very permeable to water.

The ground tissue system in the root forms two structures:

          -Cortex

          -Endodermis

The cortex is mainly parenchyma cells that are designed for food storage (primarily as starch, like potatoes)

The endodermis forms a close fitting layer of cells around the vascular tissue.

Inside the endodermis is the vascular cylinder where the xylem and phloem are located (See fig. 29.18).

The layer of cells at the outer edge of the vascular cylinder is the pericycle. This layer is just inside the endodermis. The cells of the pericycle retain their ability to divide (like the lateral meristem).

When hormones stimulate these cells they can divide to form a branch root off of the main taproot (See fig. 29.19).

SECONDARY GROWTH

The xylem and phloem in the ring of vascular bundles is divided so the xylem is toward the inside of the stem and the phloem is toward the outside of the stem.

Between the xylem and phloem is the vascular cambium (this is where the lateral meristem is located) (See fig 29.20)

During secondary growth the cells of the vascular cambium divide and add to both the xylem and the phloem.

These additions are called secondary xylem and secondary phloem respectively.

The amount of secondary growth varies during the yearly cycle, with growth occurring faster in the spring and early summer then slowing in late summer and fall.

This variation in growth rate leads to the production of growth rings. (See fig 29.25)