Knowledge is Power; Know Your Flowers (Part 2) – Flower Parts

This past March, we discussed how some plants (collectively known as angiosperms) evolved the ability to produce flowers and fruits for reproduction. We also reflected on the intricacies of inflorescence (how individual flowers are arranged on a stem) types and how these features can be used to identify plant groups or individual species. In order to identify flowering plants, you must first be familiar with floral anatomy and the terms used to describe these plant parts. While this can often be tedious, focusing on the intricacies of flowers and flower parts can also be fascinating and can teach us about a species’ ecology.

I think it is important to start with a fairly comprehensive list of flower parts and the function those parts serve, to provide a better understanding of angiosperm diversity. It is essential to note that not all flowers maintain the parts or structures discussed below. Some flowers are more “simple”; while others are more “complex.” Variation among flower characteristics (color, size, quantity, etc.) helps botanists when attempting to accurately identify a species. With these considerations, let’s jump into some flower anatomy terminology.

If you remember from last month’s discussion on inflorescences, the main stem of an inflorescence is called the peduncle, and the individual flowers on an inflorescence are attached by a pedicle. A receptacle is simply where floral organs meet the main stem.

Liriodendron tulipifera (Photo credit: Ryan Davis, ACB).

Petals are modified leaves that surround a flower’s reproductive parts. Petals often aid in attracting pollinators to a flower. Petals can be referred to collectively as a corolla. A corolla can be sympetalous or polypetalous. Sympetalous refers to a corolla that is one unit, because the petals are fused at the base of the flower. Polypetalous relates to an unfused corolla, so individual petals can be discerned at the base of the flower structure.

Sepals are modified leaves that form underneath the corolla. A collective term for sepals that you will often hear is calyx. Sepals, like petals, can be fused (synsepalous) or separate (aposepalous or polysepalous). The number of sepals is often (but not always) the same as the number of petals, as sepals act to protect an unopened flower bud. When sepals look the same as petals (petaloid), petals and sepals are collectively referred to as tepals.

The corolla and calyx are non-reproductive organs. A collective term to describe these is perianth. The presence or absence of the perianth, respectively refers to chlamydeous and achlamydeous flowers. Chlamydeous flowers can further be described as monochlamydeous, where the perianth is arranged in a single whorl, and dichlamydeous, or flowers with two whorled perianths. Homochlamydeous refers to a dichlamydeous flower when both of the whorls appear similar (tepals) or heterochlamydeous, where the corolla and calyx (the two whorls) can easily be differentiated. Homochlamydeous is characteristic of monocots and heterochlamydeous is more readily found in dicots, however, this is not always the case.

Rosa palustris (Photo credit: flawildflowers.org).

Bracts are modified or specialized leaves. Bracts are similar to sepals in this regard, but differ in the fact that they are associated with reproductive structures. Bracts can vary depending on species. In some cases bracts are mistaken for petals due to their colorful, showy appearance, but bracts can also appear like small pseudo-leaves. When a plant exhibits showy bracts, they often act to draw in pollinators to the more discrete flower. Ultimately bracts act to protect the reproductive parts of the flower. When a flower lacks bracts it is said to be ebracteate. A spathe, which we discussed last month when describing skunk cabbage (Symplocarpus foetidus), is a specific type of bract.

The collective term for the male reproductive part of a flower is called the stamen. The stamen is made up of anthers and filaments. An anther is where pollen is made and a filament acts as an anchor for the anther. Another collective term for the stamen is androecium.

The collective term for the female reproductive part of the flower is called the pistil. The pistil is made up of the stigma, style, and ovary. Other collective terms for pistil are carpel and gynoecium. A stigma is the end of the female organ that receives pollen. Styles act as an anchor for the stigma, and the ovary is the enlarged basal portion of the pistil. Inside of the ovary are ovules, which once pollinated will become seeds. Carpels vary between species. Monocarpous species have only one carpel. A species with multiple carpels that have been fused into a single structure are referred to as syncarpous. Apocarpous concerns multiple free (unfused) carpels.

Robinia pseudoacacia (Photo credit: Gardeners World).

A more specialized flower part is a nectary. Nectaries are generally located near the base of the male reproductive parts. However this can vary greatly depending on species; some flowers maintain spurs which hold nectar. Nectaries function to draw in pollinator species. As a pollinator nourishes itself on nectar, it helps to transfer pollen from one flower to another. Another specialized flower part you may notice this spring is a hypanthium. A hypanthium forms when an enlargement of the receptacle occurs. When this happens, fusion of the basal portions of the calyx, corolla and stamens results in a tubular or cup-like receptacle appearance. This is commonly found in the Rosaceae (Rose), Grossulariaceae (Ribes or Gooseberry), and Fabaceae (Legume or Pea) families. Nectar usually lines the inside of the hypanthium.

Impatiens capensis (Photo credit: Wikimedia Commons)

Grasses produce spikelets (remember this inflorescence type we discussed last month?), which maintain reproductive parts, like the stamen and pistil. However, flowering parts unique to grasses are glumes, lemma, palea, and awns. Glumes, lemma, and palea are referred to as scales. Glumes are the lowermost scales on a spikelet, and they are similar to a sepal or bract in the way they serve to protect the lemma and palea. The palea is sandwiched between the main stem (in grasses the pedicel transitions into rachilla where the spikelets sit) and the lemma. The palea and the lemma aid in protecting grasses’ reproductive organs which only appear briefly when fully mature (some grass species are self-pollinated and reproductive parts never appear). The tips of the lemmas will morph into awns. Awes are needle-like, hair projections that aid self-cultivation (once the seed is wind dispersed the awn helps the seed maintain contact with soil for germination).

Now that we have a better understanding of flower anatomy and how flowers function for plant reproduction, we can dive into a few of the important patterns you might find while closely observing flowers this spring and summer.

An individual flower can be perfect or imperfect. A perfect (also known as bisexual) flower contains both male and female structures; whereas an imperfect, or unisexual, flower has only male or only female structures.

Another characteristic that clues botanists into the intricacies of flowers is the ovary position relative to the other flower parts. There are three main ovary positions to consider. Epigynous or ovary inferior refers to the flower parts situated atop the ovary. Hypogynous or ovary superior relates to the ovary lying above where the petals, sepals, and male reproductive parts attach. A perigynous ovary position relates to the flower parts attaching around or at level to the ovary. The position of an ovary in relation to the other flower parts allows botanists to form an idea of which family a plant belongs to. For example, Iridaceae (Iris) and Caprifoliaceae (Honeysuckle) families maintain epigynous ovaries; Brassicaceae (Mustard) and Papaveraceae (Poppy) families have characteristic hypogynous ovaries. A present hypanthium is often closely related to a perigynous ovary, so the Rosaceae (Rose) and Grossulariaceae (Ribes) family will have members with ovaries and flower parts at level.

Individual flowers are sometimes distinguished by their symmetry. Actinomorphic symmetry relates to radial symmetry, meaning the flower can be divided into two equal halves along multiple planes. Rosa palustris, or swamp rose, models actinomorphic symmetry. Zygomorphic symmetry relates to bilateral symmetry, meaning that the flower can be divided into two equal halves along only one plane. All flowers in the family Fabaceae are zygomorphic; Robinia pseudoacacia, or black locust, is one that the Alliance often plants in our buffers. Flowers can also be asymmetric.

Mimulus ringens (Photo credit: Prairie Restorations, Inc.).

This spring and summer when you’re identifying PA natives, don’t be Impatiens, use this newfound toolbox the Knowledge Is Power; Know Your Flowers series provided! All puns aside, I hope this exploration of the diversity of flowers is as valuable to you readers, as it was to me while writing!