The Four Different Types of Floral Organs Are Initiated as Separate Whorls

Floral meristems initiate four different types of floral organs: sepals, petals, stamens, and carpels (Coen and Carpenter 1993). These sets of organs are initiated in concentric rings, called whorls, around the flanks of the meristem (Figure 24.3). The initiation of the innermost organs, the carpels, consumes all of the meristematic cells in the apical dome, and only the floral organ primordia are present as the floral bud develops. In the wild-type Arabidopsis flower, the whorls are arranged as follows:

• The first (outermost) whorl consists of four sepals, which are green at maturity.

The second whorl is composed of four petals, which are white at maturity.

• The third whorl contains six stamens, two of which are shorter than the other four.

• The fourth whorl is a single complex organ, the gynoecium or pistil, which is composed of an ovary with two fused carpels, each containing numerous ovules, and a short style capped with a stigma (Figure 24.4).

(A) Longitudinal section through developing flower

(A) Longitudinal section through developing flower

Vascular tissue

FIGURE 24.3 The floral organs are initiated sequentially by the floral meristem of Arabidopsis. (A and B) The floral organs are produced as successive whorls (concentric circles), starting with the sepals and progressing inward. (C) According to the combinatorial model, the functions of

Stamen Carpel Petal Sepal

Vascular tissue

(B) Cross- section of developing flower showing floral whorls

(C) Schematic diagram of developmental fields

(B) Cross- section of developing flower showing floral whorls

Stamen Carpel Petal Sepal

Whorl 1:

sepals-

Whorl 2:

petals—

Whorl 3:

stamens

Whorl 4:

carpels-

(C) Schematic diagram of developmental fields

FIGURE 24.3 The floral organs are initiated sequentially by the floral meristem of Arabidopsis. (A and B) The floral organs are produced as successive whorls (concentric circles), starting with the sepals and progressing inward. (C) According to the combinatorial model, the functions of each whorl are determined by overlapping developmental fields. These fields correspond to the expression patterns of specific floral organ identity genes. (From Bewley et al. 2000.)

Stigma Style

Ovary

Stigma Style

Ovary

FIGURE 24.4 The Arabidopsis pistil consists of two fused carpels, each containing many ovules. (A) Scanning electron micrograph of a pistil, showing the stigma, a short style, and the ovary. (B) Longitudinal section through the pistil, showing the many ovules. (From Gasser and Robinson-Beers 1993, courtesy of C. S. Gasser, © American Society of Plant Biologists, reprinted with permission.)

Transmitting tissue

Ovules

FIGURE 24.4 The Arabidopsis pistil consists of two fused carpels, each containing many ovules. (A) Scanning electron micrograph of a pistil, showing the stigma, a short style, and the ovary. (B) Longitudinal section through the pistil, showing the many ovules. (From Gasser and Robinson-Beers 1993, courtesy of C. S. Gasser, © American Society of Plant Biologists, reprinted with permission.)

Transmitting tissue

Ovules

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