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Lesson 3: Stems & Roots
Unit 5 Plants
Functions of Stems
Economic value: wood products/lumber
Connect the vascular tissue in the leaves to the vascular tissue in the roots, allowing water and dissolved substances to be transported throughout the body
Function of Stems
Raise and support the leaves (maximizing photosynthesis) and reproductive organs (maximizing the likelihood of being pollinated and producing fruits and seeds)
Stem Structures – Herbaceous Plants
Herbaceous Plants: plants that have stems that do not
contain wood Relatively pliable stems Stems carry out photosynthesis Thin epidermis
Anatomy of Herbaceous Stems Vascular tissue in herbaceous plant stems are
arranged in vascular bundles Vascular bundles contain xylem and phloem
Xylem is always closer to the centre of the stem Phloem is always closer to the outside of the stem In monocots:▪ Vascular bundles found through out the ground tissue
In dicots:▪ Vascular bundles form a ring
Monocot Herbaceous Stem (ex: corn)
Eudicot Herbaceous Stem (ex: alfalfa)
Stem Structures – Woody Plants
Woody plants: Stems that contain wood Woody stems are relatively hard Have bark Usually do not carry out photosynthesis
Anatomy of Woody Stems
Wood stems grow thicker every year due to the presence of the vascular cambium
Vascular Cambium: the meristematic cell layer in the vascular tissue that divides to form new xylem and phloem tissue
Xylem is on the inside of the vascular cambium
Phloem is on the outside
Wood – Sapwood and Heartwood
Wood is many layers of xylem tissue cells
Sapwood is the younger xylem through which water and minerals are transported to the leaves
Eventually, the older xylem fills up with resin and oils and they no longer conduct water forming the heartwood which is very rigid and helps support the tree
https://www.youtube.com/watch?v=jrXSHLyAQeQ
Bark consists of all tissues found outside the vascular cambium Phloem▪ transports sugars made in the leaves
throughout the plant. Cork cambium▪ meristematic tissue that produces cork
Cork▪ the tough, outer layer of the tree that
prevents water loss from the stem
Growth Rings
Growth only happens in the spring and summer
In spring, vascular cambium grows rapidly, producing large xylem cells that have relatively thin walls that form a layer of lighter-coloured wood
In summer, fewer xylem cells are produced and they have thicker cells walls that form a layer of darker-coloured wood
The spring and summer wood together forms one growth ring
Roots
Function of Roots: Anchors the plant Keeps it upright Absorbs waters and nutrients (0ther
than carbohydrates) Store water and carbohydrates
Root Systems
Taproot: a root system composed of a large, thick root; can have smaller lateral roots
Fibrous: a root system made up of many small, branching roots
General Structure of Roots
Tip of root contains the root cap and a meristem
Root Cap: mass of cells that form a protective covering for the meristem at the root tip; allows the root to penetrate the soil with minimal damage
Roots
Root hairs project out of the epidermis and increase the surface area allowing for greater water and nutrient absorption
Root cortex is a region of parenchyma cells beneath the epidermis where carbohydrates can be stored and water can be transported fro the epidermis to the xylem
Endodermis: is the innermost layer of cells in the root cortex
Endodermis
The endodermis cell walls are wrapped with a wax-like substance forming a continuous barrier called the Casparian strip. The Casparian strip prevents substances from passing through the spaces between the endodermal cells
Monocot and Eudicot Roots
Vascular tissue is arranged differently in monocot roots and eudicot roots
Homework Questions