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WHY HAVE FLOWERS?

Why not? GYMNOSPERMS, non-flowering plants like conifers and cycads, don’t have flowers. And they do just fine. After all, white pine is one of the most abundant species in Vermont. But if you look at a forest another way and take a survey of SPECIES RICHNESS of the trees in Vermont you would find conifers drastically outnumbered by maybe 7 to 1. If you included shrubs and non-woody flowering plants the number would be even more skewed, something like 100 to 1. So clearly the flowers of ANGIOSPERMS confer some advantage to this clade.

The primary role of a flower is to facilitate pollination by relying on animals and not wind as the vector for transferring pollen to the female GAMETOPHYTE. Gymnosperms rely primarily on chance, waiting for wind to blow through their bows, shaking pollen loose from male cones by the billions, washing these tiny grains out across the landscape in the hopes that one might land on the sticky receptacle of a female cone (PDF). I imagine billions of tiny bachelor pollen grains commiserating in gutters and puddles: “If I could’ve just floated a little farther…” Flowering plants made a deal with animals 140 million years ago: you take our pollen directly to the female gametophyte (now housed in a pretty little flower signaling the trade), and in exchange for your efforts, we’ll give you a rich nectar brew of sucrose, fructose, glucose, and sweet smelling chemicals (or at least sweet to the pollinator). Today, Angiosperms account for about 80% of plant diversity!! Here we’ll look more closely at the how and why of the effectiveness of a flower.

Gymnosperm “Flowers”
(Click image for gallery of examples)

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Angiosperm Flowers
(Click image for gallery of examples)

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[foogallery id=”19117″]

Click above for gallery of labeled tree flowers

Anatomy of a flower

A flower is the reproductive structure found in Angiosperms, and has four basic components. In COMPLETE flowers, all 4 whorls are present (a whorl just refers to the collection of that structure on a flower; e.g. the 5 petals on an apple blossom are a whorl of petals):

Structure Whorl Description
Stamen Androecium The male, pollen-producing sexual organ composed of a FILAMENT and an ANTHER
Carpel Gynoecium The female, ovule producing sexual organ, composed of a STIGMA, STYLE, OVARY, and OVULE(s). The set of 1 or more fused carpels is a PISTIL. The gynoecium is the set of all pistils within the flower.
Petal Corolla Modified leaves, typically brightly colored, that surround the reproductive organs
Sepal Calyx Modified leaves, typically green, that protect the flower before it unfurls and support it after it opens. Occasionally retained after fruiting (like the papery sheath on tomatillos)

The flower also consists of 2 non-whorled structures: the PEDUNCLE (or PEDICEL in an inflorescence) and the RECEPTACLE. The peduncle is the shortened stem The receptacle is the thickened part of the peduncle from which the flower parts develop. It is often located below the ovary (hypogynous, as in hypo: below + gynous: in reference to the gynoecium), but sometimes the ovary can be nested partly (perigynous) or fully (epigynous) within the receptacle.

Not all flowering plants have all 4 whorls on each flower. Any flower missing one or more whorls is INCOMPLETE. A male flower, which lacks at least the pistil, is called a staminate flower. Technically, the flower is part of the SPOROPHYTE and isn’t “male” since it’s the pollen grain (the male GAMETOPHYTE) that contains the male sex cells. If a whorl is missing, it means that it’s not necessary given the ecology of that tree. Wind pollinated flowers, for example, don’t need to visually attract the wind, and so tend to have greatly reduced or no petals at all. In some flowers the sepals are so modified that they look indistinguishable from petals (like the 3 outer “petals” on daylilies. When you can’t tell the difference between the two, we call the two whorls tepals.

Types of inflorescences

We’ve talked about the individual unit of a flower, but plants often develop a cluster of flowers on a branch. This cluster of flowers, called an INFLORESCENCE, can take a variety of appearance, but the type is characteristic to a particular species. As most trees are wind pollinated, by far, the most common inflorescence type in trees is the catkin. In trees and shrubs that produce big sugary fruits, like black cherry, hawthorn, viburnums, elderberry, honeysuckle, etc. the inflorescence structures are much more variable.

  • CATKIN: Birches, aspens, alders, walnuts

Sexes in Trees

A reminder that a flower can be PERFECT, having both male and female whorls, or it can be have either male or female parts, called IMPERFECT. So that’s within a flower, but individual trees of a species can have either all perfect flowers or all imperfect flowers. And for those that have imperfect flowers, the tree might have all male or all female or some combo of male and female flowers. Here’s the lingo for the sex of a tree:

  • MONOECIOUS: (from mono: one + eikos: house, so one house for both sexes) either the tree has perfect flowers or the tree has imperfect flowers but has both male and female flowers on the same tree. Typically, the tree has one sex on one tier of the tree and the other above or below.
  • DIOECIOUS: a tree with imperfect flowers, and with only one sex of flowers on any individual plant. Female redcedar  takes on a bluish cast that can be seen from a distance from all the blue “fruits”

A few trees are polygamodioecious, meaning that they are primarily either male or female but can have a few flowers of the opposite sex on the tree.

In a cool and bizarre turn of events, one of the oldest and largest trees is a yew. Rather recently some of its branches switched sexes from female to male.

The birds and the bees as it relates to trees

Pollination
Coming soon…

Fertilization
Coming soon…

Below is hands down the best video on the subject I’ve found. Enjoy!

Pollination Syndromes

Similar to dispersal syndromes with seed morphology, pollination syndromes are patterns of a suite of morphological flower characteristics that repeat themselves given a specific type of pollinator.

  • Wind Pollinated:
  • Bat pollinated:

Resources

  • Coming soon