The green leaf-like sepals compose the outermost whorl of modified leaves on this rose flower. The next whorl inside the sepals is composed of colorful pink petals on this 'Betty Boop" rose.

Numerous stamen with yellow filaments and dark brown anthers are in a whorl inside the whorl of pink petals. The last and innermost modified leaf structure is the female reproductive structure, the pistil or carpel. In this photo numerous pistils can be seen at the very center of the flower.

 stigma -

 

 

                  style -

 

 

 

 

     ovary -

Pistil of the princess flower (Tibouchina urvilleana). It is composed of a stigma at the top (where pollen from the male anther enters), a stalk called the style, and the ovary with the ova (egg) inside. The ovary will develop into the fruit with seeds inside.

The Angiosperm flower

The stuff of poetry and passion, the flower certainly needs no introduction. Botanically, the flower is a fascinating structure designed to promote pollination (mating).

The flower is composed of four nested whorls of modified leaves. The outermost whorl is typically green in color. The individual units are called sepals, all of the sepals together is called the calyx.

The next whorl (the 2nd whorl), as illustrated in the photo above, are the petals. They are located inside the sepals, and taken together they are called the corolla.

The attractive colors and markings on the petals of flowers are designed to attract pollinators (birds, bees and other insects) to carry pollen from the male stamen to the female pistil so that fertilization of the ova can occur.

Some flowers will have a single whorl of petals, others will have more than one whorl. In plants having flowers with a single whorl of petals, the flowers are said to be "single". If some of the stamen are modified into petals, then there will be multiple whorls of petals and fewer stamen. These flowers are called "double" or "semi-double".

Not all flowers will have all four floral parts. Some plant species produce flowers without petals ("apetalous"). Oak, willow and alder are examples.

Inside the petals is the third whorl of modified leaves, the male reproductive structures called stamen. Each stamen (see photo below) is composed of a stalk (filament), on top of which is the anther where pollen (equivalent to sperm) are produced

Flowers are a valuable tool for identifying and classifying plants. One important characteristic that is relatively easy to see is the pistil or carpel. Some flowers have one carpel, that is, one stigma, style and ovary. Some flowers have numerous carpels fused into one structure which is then more properly called a pistil.

To illustrate, if we take the Tibouchina pistil illustrated above and cut it in half:

Here you can see five wedge-shaped ovaries (each chamber is an ovary) which tells you that this pistil is composed of five fused carpels. To summarize, the pistil of the princess flower is actually composed of five fused ovaries, five fused styles (the purple structure), and five fused stigmas. This example also illustrates the value of cutting open flower parts and fruit (mature ovaries) if you want to know how many ovaries are present - this can be a valuable identification tool which we will revisit when we cover fruit.

Some species produce flowers that lack the male or female parts. They produce two types of flowers: male and female. If both male and female flowers are on the same plant, the term "monoecious" (Greek for "one house") is used. Oak trees are monoecious with separate male flowers and female flowers on the same tree. If a species produces male flowers on one plant and female flowers on another, the term "dioecious" is used ("two houses"). English holly is a good example of a dioecious species; if you want nice red berries on your holly, make sure it is a female plant since male plants don't produce fruit.

 

STUDENT RESEARCH QUESTIONS:
11. If oak trees (Quercus) flowers are apetalous, how does pollination occur?
12. Does the Kwanzan cherry (Prunus serrulata 'Kwanzan') have single, semidouble or
     double flowers? (Try search: Prunus serrulata 'Kwanzan' image)
13. Does the Akebono cherry (Prunus x yedoensis 'Akebono') have single, semidouble or
     double flowers?

 

BACK TO COURSE SYLLABUS

ON TO NEXT TOPIC: The Angiosperm inflorescence

                    What looks like one stigma on the
                    outside is actually five fused stigmas -

                                             What looks like one style on the
                                             outside is actually five fused styles -

 

                                What looks like one ovary on the
                                outside is actually five fused ovaries-