Anatomical Events Leading to Flower Initiation

• Largest nucleoli occur in nuclei of mantle and corpus cells in meristematic vegetative cells
• Nucleoli of all cells increase in size after flower induction
  • Mantle cells increase in number and are larger than corpus cells
• Rate of cell division increases in the apical region
  • Surface cells elongate and divide anticlinally
  • Second and third layers divide periclinally
  • Fourth layer cells divide anticlinally and periclinally
• Rate of floral initiation decreases
• Floral primordia are differentiated into bracts rather than leaves
• Apical dominance in reduced
• Primordia grow and differentiate into corolla tube, stamen, pistil and ovules depending upon the hormonal balance
• Exposure to unfavorable events can prevent further or alter development
  

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• Development of the ovary (Megasporogenesis)
  • An ovary may contain one or more ovules
  • Each ovule arises separately from the placenta
  • Each ovule has one large cell (archesporial cell)
  • Mitotic division = megaspore mother cell  (MMC) and parietal cell
    • Two meiotic divisions of MMC = four haploid megaspores
    • One megaspore develops into an embryosac
      • Three mitotic divisions = 8 nuclei
      • Nuclei = 1 egg, 2 synergids, 2 polar nuclei, 3 antipodal cells

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• Development of the pollen  grains (Microsporogenesis)
  • Microspore mother cells (MMC) form within the sporangia
  • MMC undergo meiosis (microsporogenesis) = 4 microspores
  • Microspores undergo mitosis (microgametogensis)
  • Microgametogensis = pollen grains (microgametophytes)

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Pollination and Seed Development

• Developmental stages:
  1. Formation of stamens and pistils
  2. Anthesis (opening of the flower)
  3. Pollination (transfer of pollen grains)
  4. Fertilization (egg cell and polar nuclei)
  5. Differentiation (fertilized egg into embryo, endosperm and seed coat)
  6. Accumulation of stored food
  7. Maturation

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Floral Morphology

• Typical angiosperm flower:
  • Petals, sepals, stamens and pistil
    • Collectively, petals = corolla
    • Collectively, sepals = calyx
  • Female parts - sigma, style and ovary
    • Ovary may be composed of one or more carpels
      • Outermost wall is the pericarp
• Types of flowers
  • Complete - petals, sepals, stamens and pistil
  • Incomplete - lacking any of above
  • Perfect - male and female parts
  • Imperfect - unisexual flower
• Species dependent floral types
  • Monoecious - male and female flowers separated on same plant
    • corn
  • Dioecious - unisexual flowers on separate plants
    • holly

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Fruit Development

• Fruit - a mature or ripened ovary
• Seed - a mature ovule consisting of an embryonic plant combined with stored food and a protective coat
• Pericarp layers:
  1. Exocarp = outer layer
  2. Mesocarp = middle layer
  3. Endocarp = inner layer
     • Relative development of the above layers determine the fruit structure and morphological differences among species

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Fruit types

1. Pseudocarpic fruit - one or more ripened ovaries attached or fused to modified bracts or other nonfloral structures
   • burdock, sandbur
2. Multiple fruit - ovaries of more than one flower
   • fig, mulberry, pineapple
3. Aggregate fruit - several ovaries of a single flower
   • strawberry, raspberry, blackberry
4. Simple fruit - derived from a single pistil
   • Fleshy fruits  - fleshy or leathery pericarp
     1. Berry - fleshy pericarp
        • grape, tomato
     2. Pepo - hard rind without internal separations
        • watermelon, cantaloupe, cucumber
     3. Pome - thick fleshy exocarp and papery endocarp
        • apple, pear
     4. Drupe - stony endocarp, thick leathery or fleshy mesocarp and thin exocarp
        • cherry, coconut, peach
     5. Hesperidia - berrylike fruits with papery internal separations
        • orange, lemon, lime, grapefruit
   • Dry fruit - thin pericarp that is dry at maturity
     1. Dehiscent fruit 
        • Legume - simple pistil that splits open along two sutures at maturity
          • bean, pea, soybean
        • Follicle - compound pistil that splits open along one suture at maturity
          • milkweed, larkspur
        • Capsule - has a compound pistil that splits open along one of  four ways
          • iris, plantain, yucca, poppy
     2. Indehiscent fruits
        • Achene - seed is attached to pericarp at one point and may be loose inside
          • sunflower, dock
        • Caropsis - entire seed coat is fused with the pericarp
          • grasses
        • Samara - pericarp develops a thin, flat, winglike appendage
          • ash, elm, maple (double samaras)
        • Nut - one seeded fruit from a compound pistil with a hard, tough pericarp
          • acorn, chestnut
        • Schizocarp - two fused carpels separating to form one-seeded mericarps at maturity
          • carrot

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Floral Taxonomy

• Arrangement
  • Inflorescence - arrangement or structure of the flower
  • Peduncle - main stalk of the inflorescence
  • Pedicels - lateral stalks supporting individual flowers
• Types of Inflorescence
  1. Determinate- axis terminates as a flower
     • Solitary flower - simplest expression
     • Simple cyme - lateral flowers develop later than the terminal flower
       • chickweed
     • Compound cyme - has secondary branching with each lateral unit as a simple cyme
       • bouncing bet
  2. Indeterminate - pedicels arise laterally on a long peduncle
     • Raceme - the basic indeterminate inflorescence
       • pennycress
     • Panicle - lateral branches produce flower-bearing branches instead of single flowers
       • oats
     • Spike - flowers along the peduncle are essentially sessile or stalkless
       • wheat
     • Catkin - a modified spike with a unisexual flower
       • red alder
     • Spadix - a spike covered by a spathe
       • skunk cabbage
     • Corymb - lower pedicels are successively longer than the upper ones on the peduncle
       • bitter cherry
     • Umbel - similar to a corymb except that the lateral branches originate from the same location on the peduncle
       • pennywart, wild carrot
     • Head - peduncle and pedicels are tightly clustered and are surrounded by a group of bracts
       • sunflower

References:

Principles of Seed Science and Technology, 3rd Ed., L.O. Copeland and M.B. McDonald. Chapman and Hall, New York, 1995.

Seeds Handbook, B.B. Desai, P.M. Kotecha and D.K. Salunkhe. Marcel Dekker, Inc., New York, 1997.