Plant stems

• The stem is an example of a plant organ
• The primary function of the stem is to support, to hold the leaves in the best position for obtaining sunlight for photosynthesis. Stems also support the flowers un a way to maximise the likelihood of pollination.
• Stem must provide flexible support as they need to bend to endure the forces of the weather but still have the strength to stay upright
• The second major function of stems is the movement of materials around the plant. Products of photosynthesis are carried from the leaves to other parts of the plant that might need them and water moves through the stems up to the leaves with mineral ions
• Most stems are green because they contain chlorophyll. They carry out a small amount of photosynthesis
• The outer layer of the stem is the epidermis, which does not provide support but protects the cell beneath it.
• Much of the stem is packing tissue which consists of the most common type of plant cell, parenchyma. Parenchyma are unspecialised cells that can be modified in several ways so they become suitable for storage and photosynthesis
• Collenchyma cells are found around the outside of the stem. They have thick cellulose walls which gives them strength. They gave plenty support but remain living and can stretch as the plant grows to provide flexibility
• 1. Sclerenchyma
  2. collenchyma
  3. epidermis
  4. xylem
  5. parenchyma
  6. phloem
  7. cambium

  
• Sclerenchyma is a type of modified parenchyma (packing) tissue found in plant cells. As the plant grows, it develops to support the increasing weight of the upper part of the plant. Sclerenchyma tissue is found around the vascular bundles. Sclerenchyma cells all have strong secondary walls made of cellulose microfibrils positioned at right angles to each other. Lignin is deposited on the cell walls of these fibres in a spiral or ring pattern, and this makes the fibres strong but flexible
• When fibre is lignified, the cell contents due be ages water cannot pas through lignin, and so the fibres become hollow tubes
• Sclerenchyma can also become completely impregnated with lignin and form sclereids. These very tough cells may be found in groups throughout the cortex of the stem or individually in plant tissue
• The main transport tissues are the xylem and phloem and they are found in vascular bundles throughout the plant
• Xylem tissue carries water and dissolved mineral ions from the roots to the photosynthetic parts of the plant. The movement in the xylem is always upwards and most xylem cells are dead
• Phloem is a living tissue mass of phloem cells which translates the dissolved products of photosynthesis (sucrose) and amino acids from the leaves to where it is used for growth or storage as starcg. The flow in the phloem can go both up or down a plant
• Cambium is a layer of unspecialised cells which divide, giving rise to more specialised cells that form both the xylem and the phloem
• The xylem starts as a living tissue. The first xylem the plant makes is called the protoxylem. It can stretch and grow because the walls are not fully lignified. Increasing amounts of lignin means the cells stop growing and are impermeable to water and other substances. The tissue becomes more supportive and stronger, but the contents of the cells die. This lignifies tissue is called the metaxylem
• Water and minerals are transported frol the roots to the leaves and shoots in the transpiration stream. Water moves out of the xylem through specialised pits in the walls of xylem vessels
• Materials in the phloem are transported both up and down the stems in an active process called translocation
• Phloem cells do not become lignified and are living. The walls between the cells become perforated creating specialised sieve plates, and the phloem sap flows through these plates.
• Mature phloem cells have no nucleus. They survive because they are closely associated with cells called companion cells. The companion cells are very active cells that all have the normal organelles, and they are linked to the sieve tubes by many plasmodesmata
• The cell membranes of companion cells have many infoldings that increase surface area for increased transport of sucrose. They also have many mitochondria to supply the ATP needed for active transport
• 1. epidermis
  2. parenchyma
  3. root hair
  4. cambium
  5. xylem
  6. phloem
  7. pericycle

  
• 1. Xylem
  2. Phloem
  3. Collenchyma
  4. Upper waxy cuticle
  5. Lower waxy cuticle
  6. Spongy mesophyll
  7. Palisade mesophyll