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Botany 101

Flower Forms

Even a casual glance at the flowers in your garden will convince you of their diversity of form. This characteristic diversity in flower forms is a very important factor in plant identification. Flowers vary in the number of their floral parts, but they also vary in other ways relative to their parts:

  1. Variations in the degree to which floral parts are united
    • Gamopetalous - petals united to form a tubular or rotate corolla with the united part known as the tube and the spreading or flat part known as the limb . These flowers take different forms as:
      • Funnel-form - the tube gradually widens upward and flares into the limb without any particular point of demarcation. Examples include Nerium oleander (oleander) and Ipomoea carnea (bush morning-glory).
      • Rotate - the tube is short and the limb is flat and circular. Lycopersicon esculentum (tomato), Sambucus canadensis (elderberry) and Vinca minor (creeping vinca) are examples.
      • Urn-shape (urceolate) - broad tube and slightly recurved, short limb, as in Vaccinium spp. (blueberry).
      • Salver-form - slender tube and an abruptly widened, flat limb. Phlox drummondii (annual phlox) and Catharanthus roseus (periwinkle) are examples.
      • Gamosepalous - flower with united sepals. Catharanthus roseus (periwinkle) and Hibiscus rosa-sinensis (hibiscus) are examples.
      • Polypetalous - petals of corolla composed of separate parts. Magnolia grandiflora (southern magnolia), Camellia japonica (camellia, and Rosa spp. (rose) are examples.
      • Polysepalous - sepals of calyx composed of separate parts. Magnolia grandiflora (southern magnolia) and Rosa spp. (rose) are examples.

       

  2. Variations in placement of floral parts on the receptacle
    • Hypogynous (hi-poj'i-nus) - sepals, petals, and stamens are attached to a convex or conical receptacle at the base of the ovary. Sepals are arranged in the outermost or lowest layer, followed by petals and stamens, with carpels or ovary innermost. The ovary is called superior and the perianth is inferior or hypogynous. Lycopersicon esculentum (tomato) is an example.
    • Perigynous (pe-rij'i-nus) - sepals, petals, and sometimes stamens borne on the edge or margin of the receptacle so that they appear to form a cup around the pistil. Prunus persica (peach) is an example.
    • Epigynous (e-pij'i-nus) - sepals, petals, and stamens appear to arise from the top of the ovary. The concave receptacle not only surrounds the ovary, but is fused with it. In this case, the ovary is called inferior and the perianth is called superior or epigynous. Malus pumila (apple) is an example.
  3. Variations in the number of subdivisions of each of the four regular parts
The number of sepals and petals is three or multiples of three of each in the monocotyledonous plants, four or five of each in the dicotyledonous plants, and reduced to none in some plants.

There is a large and indefinite number of stamens in many flower types, but in some there is a definite number, often the same as or twice the number of petals, or even further reduced to one or two.

Some flowers have a large number of separate pistils, but in others they are more or less united to form a compound pistil, and in many there is just one simple pistil.

  1. Variations in the symmetry of flower forms
    • Regular or Actinomorphic (ak'ti-no-mor'fik) - floral parts, especially the corolla, are arranged symmetrically so that when quartered all sections are equal. Rosa spp. (rose), Camellia japonica (camellia), and Ipomoea carnea (bush morning-glory) are examples.
    • Irregular or Zygomorphic (zi'go-mor'fik) - floral parts are not arranged symmetrically, and when divided horizontally the two parts are unequal and dissimilar. There are three types of irregular flowers:
    • Papilionaceous (Peas or beans) - five petals of three distinct types. Lathyrus odoratus (sweet pea) and Lupinus spp. (lupine) are examples. The three types of petals are:
      • Standard or Banner - large petal in the uppermost part or back of the flower.
      • Keel - two usually narrow and elongated petals in front of and usually below the standard.
      • Wings - two petals placed to the right and left of the keel and more or less clasping the keel.
    • Labiate (Mints) - tube of corolla usually deeply slit into two irregular lobes, the upper lobe erect and made up of two petals, and the lower lobe spreading or open and composed of three petals. Bilabiate means two-lipped or double-lipped corolla. Salvia splendens (scarlet sage), Salvia coccinea (tropical sage) and Antirrhinum majus (snapdragon) are examples.
    • Orchidaceous (Orchids) - three sepals and three petals, with one petal, usually the lower one, modified to form the very different and variable lip. Stamens are reduced and united with the pistil to form the column . Cattleya spp. (cattleya orchid) and Phalaenopsis spp. (moth orchid) are examples.

Source: Botany Handbook for Florida, Revised Edition, Kathleen C. Ruppert, January 1999 -- This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.

 

 
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