Plant Growth (DP IB Biology: HL)

• Plant growth occurs in regions known as meristems
  • The cells in these regions are known as meristem cells
• Meristem cells are undifferentiated; they actively divide by mitosis to produce new plant tissue
• Meristems can be found in the growing tips of plant roots and shoots
  • A meristem at the tip of a shoot is a shoot apical meristem
  • A meristem at the tip of a root is a root apical meristem
  • Meristem tissue with the potential to form new side branches from the main plant stem can be found in regions known as axillary buds
    • Axillary buds are those that grow in-between the main stem and the branch of a leaf
    • The growth of the axillary buds is inhibited by the plant chemical auxin released from cells in the tip of the shoot apical meristem
    • This inhibition of axillary bud growth by the shoot apex is known as apical dominance
• Meristems can also be found parallel to the sides of a stem e.g. within the vascular bundles that contain the xylem vessels and the phloem
  • These are known as lateral meristems and enable plant stems to grow in diameter
  • Lateral means ‘from the side’
• The regions of meristem tissue in the vascular bundles are known as cambium

The growing parts of plants are known as meristems, and can be found in the shoot apex (shoot apical meristem), the root apex (root apical meristem), and the sides of the stem (lateral meristem)

Lateral meristem tissue is known as cambium. It can be found between the xylem and phloem in stems and roots.

• Plant growth at meristems can continue indefinitely and is therefore said to be indeterminate
  • Animal growth is different to plant growth in that animals grow until they reach their full adult size and then stop; animal growth is therefore determinate
  • Note that while the growth of some individual plant organs, e.g. leaves or flowers, is determinate, plants can grow an indeterminate number of these organs from their meristems
• It is often possible to grow whole new plants from just a single plant cell
  • Such plant cells are totipotent
  • In some cases fully differentiated plant cells are able to do this

• Plant stems grow longer at the tip due to cell division in the shoot apical meristem
• Small cells in the shoot apical meristem divide by mitosis to produce two identical daughter cells
  • Of the two daughter cells produced by each round of division, one remains in the meristem while the other moves away to form new tissue elsewhere
• The cells that leave the meristem grow rapidly, differentiating into different cell types which themselves give rise to different plant tissues e.g.
  • Ground meristem cells give rise to ground tissues
    • Ground tissues are any tissues that are not part of the plant's vascular system or its outer protective layers e.g.
      • Collenchyma tissue cells have thickened cell walls and form supportive tissue
      • Parenchyma tissue cells carry out essential plant functions such as photosynthesis and carbohydrate storage
  • Protoderm cells give rise to epidermal tissue
    • Epidermal tissue is a layer of cells that covers the outside of the plant, forming a barrier between the internal tissues and the external environment
  • Procambium cells give rise to vascular tissue
    • Vascular tissue transports water, minerals, and assimilates around the plant
    • The vascular tissue contains the xylem and phloem
• Cell differentiation is determined by chemicals called hormones in the plant tissues
• Leaves develop from structures called leaf primordia at the sides of the shoot apical meristem

Mitosis at the shoot apical meristem produces cells which differentiate to form different plant tissues.

• Plant growth can be affected by factors in the external environment such as light, gravity, water, and the presence of objects
• These growth responses are known as tropisms
  • Tropisms can be towards a stimulus; positive tropisms, or away from a stimulus; negative tropisms
  • Tropisms enable plants to maximise their chances of survival e.g.
    • Growing towards light ensures a maximum rate of photosynthesis
    • Growing away from or towards gravity ensures that seedlings grow the right way up
    • Growing towards water enables roots to maximise their water uptake
    • Growing up and around an object may allow a plant to gain height quickly and so maximise light absorption for photosynthesis
• Tropisms are regulated by chemicals produced in plants known as plant hormones
• Examples of tropisms include
  • Phototropism
    • Plant response to light
    • Plant stems grow towards light; this is positive phototropism
  • Gravitropism
    • Plant response to gravity
    • Plant stems grow away from gravity; this is negative gravitropism
    • Plant roots grow towards gravity; this is positive gravitropism
    • Gravitropism is also known as geotropism

Plants can respond to environmental factors such as light and gravity; these responses are called tropisms and they can be either positive or negative.