Stems The plants highway of life!

The Function of Stems Provide support to plant –Withstand the forces of gravity Transport water from roots to shoots Transport sugars from sources to sinks

Movement of Water Where does water come from? –Soil How does it get to the leaves and flowers? –Absorbed by roots, transported by xylem up the stem and into the leaf veins Does this happen by magic? –NO! Two major forces are at work to overcome the pull of gravity and transport water up the stem.

Absorption of Water by the Roots Root hairs –Tiny outgrowths from the epidermal cells –Absorb water and minerals that exist between soil particles –May associate with fungal hyphae assembling a mycorrhizae that increases SA Root Pressure Push –At night epidermal cells and ground tissue of the roots use ATP to transport minerals from cell to cell leading to the xylem –Water follows via osmosis –The large amount of minerals and accompanying water creates a positive pressure in root xylem –Pushes water up roots and into stem

Hairy Roots

Getting Hydration from Root to Tip Transpiration-pull –Like drinking through a straw –As water evaporates out of leaves, transpiration pulls water up the stem to replace the water that was lost –The higher the transpiration rates, the stronger the pull –Occurs during the day, via cohesion and adhesion of water molecules Cohesion similar molecules stick to one another (-) O of one H 2 O sticks to (+) of another H 2 O Hydrogen bond Travel up as a chain Adhesion Unlike molecules stick together Water molecules stick to the cellulose of xylem cell walls

Xylem Cells DEAD! Tracheids and Vessel Elements overlap to form tubes Pits in the cell wall of both allow water to move freely through cells and up tubes.

Nourishing the Plant Function of Phloem Moves sugars from sources to sinks –Sources: sites where sugar is made or released –Sinks: sites of sugar storage

How does it work? Phloem tissue –Moves sugar, other organic matter –Process called Translocation –Moves through chains of cells called Seive- tube members End of each cell looks like a seive Alive –May lose cellular organelles, therefore companion cells are close by to take over cellular functions

The Pressure Flow mechanism What do you know about Phloem? –Sugar? –Water?

Differences Stem Between Monocots vs. Dicots Vascular bundles arranged differently in stem of monocots and dicots –Monocots: scattered –Dicots: arranged in a ring

Try questions on pg 407