StemsStems have many functions 1. Support the leaves (hold them up) 2. Transport Water and Sugars 3. Provide Storage
Basic Stem Structures – end of the internode where one or more leaves are attached – Stem segments
The Function of Herbaceous Stem Tissues • Epidermis – outer protective layer • Cortex – ground tissue….composed of parenchyma cells (storage cells) • Sclerenchyma – strength tissue…. prevents xylem and phloem from kinking • Vascular Bundle – made up of the xylem, phloem and sclerenchyma • Xylem– transports water • Phloem – transport sugars
Woody Trunk X-section Sapwood Heartwood
Epidermal Hair Functions • Provides protection from evaporation by creating a protective, immobile surface barrier on the leaf that is that is considerably thicker than a smooth, hairless leaf. • A leaf with hairs loses much less moisture through surface evaporation in windy weather and thus less water is drawn from the leaf. • Provides the plant with protection from parasites. • Hairs can either contain antibodies to protect the plant from herbivores, as in the case of the stinging nettle, • or, as seen in tobacco plants, leaf hairs secrete substances that prevent fungal infection.
Translocation Translocation is the movement of sucrose and other organic materials from one place (a source) to another (the sink) within the plant body, primarily through the phloem. Concentrations drive this process.
Translocation begins in any plant location where sucrose (or the other organic solute) is in high concentration - usually the palisade or spongy mesophyll cells in the leaf. • Sucrose is manufactured by chloroplasts in these cells, so is in greatest concentration here. • There is a lower sucrose concentration in the sieve tubes (of the phloem). • This concentration or pressure difference causes the sucrose to diffuse toward and into the phloem of the leaf and into the phloem of the stem (where there is even less sucrose).
Translocation Sucrose moves up and down the stem in the phloem, in the direction that has the lower concentration of sucrose. • Cells "burn" sucrose as fuel for respiration; • Cells need sucrose to construct cellulose (a key component of wood); • Cells store sucrose for later use, withdrawing sucrose from the stem and lowering the sucrose concentration.
Transportationof water through the Plant The main water transportation tissue in plants is the Xylem. Xylem is made up of Tracheids and Vessel Members.
Osmosis- water moves into the stele and into the xylem Capillary Action- moves the water up through the xylem Cohesion- water sticks to other water molecules Adhesion- water sticks sticks to other molecules, like the tubes formed by active xylem cells Transportationof water through the Plant