Specialized Stems and Roots

• specialized vegetative structures:

  • bulb
  • corm
  • tuber
  • tuberous stem
  • tuberous root
  • rhizome
  • pseudobulb
• Specialized vegetative structures are mostly herbaceous perennials whose shoots die down at the end of a growing season
• specialized vegetative structures functions:

  • storage of food, nutrients, and water
  • survival during adverse environmental conditions
  • clonal regeneration
• geophytes: plants that survive periods of adverse environmental conditions as underground storage organs
• Geophytes are never physiologically dormant
• separation: a type of clonal propagation that uses detachable structures on the plant as propagules
• division: a type of clonal propagation that involves cuttings or dividing the plant into sections with stems and roots
• Bulbs consist of a short modified stem, roots or root primordia, axillary buds and flower apex enclosed in fleshy leaves (scales) modified for food storage
• Bulbs are underground structures produced mainly by monocots
• basal plate: short or compressed, thickened stem of a bulb
• bulb scale: leaves of a bulb
• Outer bulb scales are generally fleshy and contain food reserve material
• Inner bulb scales function less as storage organs and more leaflike
• tunicate bulbs: fleshy scales in continuous, concentric layers
• tunic: dry and membranous outer bulb scales of tunicate bulbs
• Tunic provides protection from drying and mechanical injury to bulb
• Tulip bulb: has only true, leaf-like scales inside the bulb with leaves produced on the flowering or vegetative shoot
• Daffodil bulb: has both expanded leaf bases and true scales
• Amaryllis bulb: has expanded bases of leaves that are used for food storage; there are no scale leaves
• Non tunicate bulbs: have scales that are separated and attached to the basal plate
• Non tunicate bulbs do not have a dry covering so they can be easily damaged and must be kept moist
• Adventitious root primordia are present on dormant stored bulbs
• Geophytes have both fibrous and contractile roots
• fibrous roots: roots formed as adventitious roots; absorb water and nutrients and normally function for one growing season
• contractile roots: thickened, fleshy roots that pull bulb to a deeper layer in soil; function is to adjust the depth of the bulb/corm
• dropper: a modified stem that grows deep to produce a new bulb
• bulb growth stages:

  1. vegetative stage
  2. reproductive stage
• bulb vegetative stage: the bulblet grows to flowering size and attains its maximum weight
• bulb reproductive stage:

  • induction and initiation of flowering
  • differentiation of floral parts
  • elongation of the flowering shoot
  • flowering
  • sometimes seed production
• offsets: a latent shoot that develops from the base of the main stem
• stem bulblet: a miniature bulb that forms in the axial of a bulb scale and used for propagation
• bulbils: aerial stem bulblets
• bulb stem cutting: cuttings are made shortly after flowering
• bulb leaf-bud cutting: made with a single leaf and a small heel of old stem
• bulb leaf cutting: an entire leaf is cut into a few pieces, basal end is placed below surface, bullets form on the base within a few weeks
• scaling: individual scales are directly planted in growing media, adventitious bulblets form at the base of each scale
• scooping: the basal plate of a mature bulb is scooped out with a special scoop-like device; adventitious bulblets develop from the base of the exposed bulb scales
• scoring: three straight knife cuts are made across the base of the bulb, each deep enough to go through basal plate and growing point; growing point grows into bulblets
• twin scaling: dividing bulbs into segments, each containing a pair of bulb scales and a piece of basal plate
• corms: base of the stem axis is swollen, has nodes and internodes, and is enclosed in a dry membranous tunic