Plant Families

David Beeson, May 2022

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The placing of organisms into groups some people think is a rather boring topic. I agree; yet understanding some aspects of classification makes life and studying organisms easier and rewarding.

Organisms in any group have features in common and possibly a common ancestor. I’m in a group: I follow Southampton Football Club and that is the common feature of Saints Supporters (as we are called). My ancestory was being brought up near the city of Southampton.

The PLANT KINGDOM has sub-sections (Divisions): liverworts, club mosses, hornworts, ferns, ginkgos, cycads, conifers and flowering plants (monocotyledonous and dicotyledonous groups).

(Sadly, not all botanists agree how to subdivide the plant kingdom … which makes it near impossible for everyone else.) (NOTE, if you are studying botany at degree level, stick to whatever system your professors says! She / he will be marking your examination scripts!)

Here, we will look only at the flowering plants and their FAMILIES.

When ‘in the field’ I often need to identify a plant. With most ID books using the plant family (rather than flower colour or plant size or habitat) to organise their system, knowing the family saves much effort.

Many people become confused with plant families. And, it is understandable as seldom is it explained.

The flowering plants – the Angiosperms.

These are plants that flower and have seeds produced in fruits. The seed being an immature plant consisting of a minute root, shoot, stem and one or two seed leaves – cotyledons. A fruit is produced from the ovary of the plant*. (Gymnosperms have seeds, but they are not contained within an ovary – gymnosperm = naked seeds …. as they are found on the surface of an open cone, so naked.)

[*What is the true definition of a fruit? A fruit is a mature, ripened ovary, along with the contents of the ovary.]

In my opinion, there are two sub-groups of the Angiosperms – the monocotyledonous plants (monocots) with one cotyledon in the seed, and the dicotyledonous plants (dicots for short) with two.

  • Monocots usually have leaf veins that run parallel and flower petals / sepals in multiples of threes.
  • Dicots have netted leaves and flower sepals / petals in combinations of four or five.

Plant families are decided on their flower structure. Size is of no consequence. Flower colour is of no consequence. Members of a plant family may be trees, shrubs or herbaceous, it makes no difference – it is flower structure that is crucial.

So, which floral features are important?

  • The number of floral parts i.e. how many petals or sepals?
  • Are the sepals coloured and look like the petals?
  • Do the petals all join to form a corolla?
  • Is the ovary above the junction of petals meet with the flower stem (superior ovary) or below it (inferior ovary)?
  • How many flowers are there on a flower stem and how are they organised?  
  • Etc.
The pistil is also called the carpel. Superior ovary is shown.
The receptacle can be below the female parts (Gynoecium is an alternative name for pistil / carpel), surround it or enclose it.
No one said flower structure was easy!

Firstly, recall that flowers are in layers. The bottom layer comprises the sepals; next petals, then stamens and carpels at the top. They all join to the receptacle that sits atop the flower stalk. The receptacle can surround the carpels.

Orchid flower diagram – concentrate on sepals and petals and ignore the rest.

For example, the Orchid Family.

They all have parallel leaf veins, so are monocots and seeds have a single cotyledon. There are no dicot orchids.

Floral parts (sepals and petals) are in threes, but in a specific arrangement. The three sepals are at the top and sides. Two petals form a hood, while a large petal flows down at the front. That package is only found in the orchids.

Even parasitic orchids, with no green parts and brown flowers, have that structure.

UK wild orchids and exotic orchids from Costa Rica have the same structure.

Now perhaps go to your plant ID book, your flora, and skip through the pages to see the different plant families and seek out their characteristics (Often stated in the introduction to that family).

I do not know all the families … in fact, not even near! But, the more often I ID a plant the more families I start to understand.

Remember: eBay offers secondhand botany books at almost zero cost. They do not go out of date easily. That’s where my uni-level books come from.

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