Plant form and physiology

• Plant growth is concentrated in localized regions of cell division called meristems
• Nearly all nuclear divisions (mitosis) and cell divisions (cytokinesis) occur in meristematic regions
• In a young plant, the most active meristems are called apical meristems, located at the tips of the stem and the root
• Axillary buds at the nodes contain the apical meristems for branch shoots
• Lateral roots arise from the pericycle, an internal meristematic tissue
• Zones of cell elongation are proximal to and overlap the meristematic regions
• Cells in the zones of cell elongation increase dramatically in length and width
• Cells usually differentiate into specialized types after elongation
• Primary growth is the phase of plant development that gives rise to new organs and the basic plant form
• Primary growth results from the activity of apical meristems
• In primary growth, cell division is followed by progressive cell enlargement, typically elongation
• After elongation in a region is complete, secondary growth may occur
• Secondary growth involves two lateral meristems: the vascular cambium and the cork cambium
• The vascular cambium gives rise to secondary xylem (wood) and secondary phloem
• The cork cambium produces the periderm, consisting mainly of cork cells
• Three major tissue systems are found in all plant organs: dermal tissue, ground tissue, and vascular tissue
• Gravitropism is an important plant growth response to the environment that directs shoots upward and roots downward
• Gravity sensing involves the sedimentation of dense amyloplasts within specialized gravity-sensing cells in each organ
• Auxin promotes cell elongation in shoots and inhibits it in roots, resulting in an organ tip curvature that allows it to resume growth at a predefined angle from gravity
• Root hairs are single cells originating from "trichoblast" and can reach 4-15 mm from the root cap
• Primary growth in plants occurs as the stems and roots lengthen, primarily due to cell division in the shoot apical meristem
• Secondary growth is characterized by an increase in thickness or girth of the plant, caused by cell division in the lateral meristem
• The outer layer of the cortex of many roots is thickened to form a hypodermis or exodermis
• Lateral roots arise in the pericycle, where secondary thickening starts
• Secondary growth involves the formation of a vascular cambium in the pericycle to produce secondary phloem and secondary xylem, replacing the epidermis with a periderm
• Stem anatomy consists of parenchyma, collenchyma, and sclerenchyma cells
• Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds
• Collenchyma cells provide structural support to the stem and leaves, and are alive at maturity
• Sclerenchyma cells provide support to the plant and many are dead at maturity, with two types: fibers and sclereids
• Xylem transports water and nutrients from the soil to the leaves, while phloem transports carbohydrates from leaves to roots and fruits
• Cortex in plant stems is typically parenchyma and serves for support, storage, and photosynthesis
• Parenchyma cells:

  • Most common plant cells found in the stem, root, inside of the leaf, and pulp of the fruit
  • Responsible for metabolic functions like photosynthesis
  • Help repair and heal wounds
  • Some store starch
• Collenchyma cells:

  • Elongated cells with unevenly thickened walls
  • Provide structural support to the stem and leaves
  • Alive at maturity
  • Usually found below the epidermis
  • Example: the "strings" of a celery stalk
• Sclerenchyma cells:

  • Provide support to the plant
  • Many are dead at maturity
  • Two types: fibers and sclereids
  • Both types have thickened secondary cell walls with lignin deposits
  • Fibers are long and slender
  • Sclereids are smaller-sized
  • Sclereids give pears their gritty texture
  • Humans use sclerenchyma fibers to make linen and rope