Dr M’s autumn botany class: lovely bit of Asteraceae on campus

Dr M’s second botany lesson for his MSc Plant Diversity and MSc SISS students was all about getting to know the top-twenty plant families in Britain.

Students divided themselves into smaller groups and set out to different parts of the University of Reading campus to collect material of flowering plants and to bring them back to the lab.

Once back students sorted their botanical booty into families and Dr M introduced some of the key features of three main families (Asteraceae, Fabaceae and Lamiaceae).

Amongst the plants in the students’ collections the largest family by a campus mile was Asteraceae.

At first sight the individual capitulum looks like a single flower, but pull it apart and you find numerous tiny flowers all neatly and beautifully arranged inside.

The image below is of Leucanthemum vulgare (Oxeye Daisy) click to enlarge it and to see some of the key Asteraceous components – capitulum, ray and tube florets and phyllaries:

These little flowers (known as florets) may be ray florets or tube florets (also known as ligule and disc florets) are arranged on a little “stage” or “platform”, known as the receptacle.

In each floret you can see a corolla sitting on top of the ovary or the developing or ripe seed (achene) depending what stage the plant is in.

Once the basic pattern has been appreciated it should, theoretically at least, be not too difficult to know when you have found a species in the Asteraceae.

But even so, it is striking how much variation on the basic Asteraceae theme there is, and this can lead to some mistaken identities and errors.

There are three main types and here is some of the variation on those three Asteraceous themes found on the University of Reading Campus:

(1) Some genera have both ray and tube florets in the capitulum (e.g. Bellis and Leucanthemum and other “Daisy” types, such as all of these below, note the lovely colour range!

 

(2) Other genera have only ray florets, e.g. Taraxacum (Dandelion) and related types of so-called yellow composites:

 

(3) Yet others have just tube florets, e.g. Cirsium and other  “Thistles” plus all of these:

Also, in some species the inflorescence may bear a single capitulum (e.g. Daisies and Dandelions) while in others several capitula may be grouped in a branched inflorescence (e.g. Cat’s Ear and Hawk’s Bits). In yet others there may be numerous small capitula arranged in a massed inflorescence (e.g. Mugwort and Golden Rods).

Dr M says: beware of imposters – Asteraceae look-alikes, plants which at first glance look a tad Asteraceous, but on closer inspection are definitely not!

The closest and most convincing look-alike is the Scabious family (Dipsacaceae) which also has many small flowers (florets) clustered together on a common receptacle (like Asteraceae). However, each floret has 2-4 free stamens protruding from the corolla tube, whereas in Asteraceae there are 5 stamens fused into a tube.  Dipacaceae flowers also have long calx teeth and an epicalyx to each floret, but in Asteraceae the calyx is either a pappus of hairs, or small scales or is absent, and there is no epicalyx.

A second, and less convincing, look-alike is the Clovers (in the Pea family, Fabaceae). With a dense head of small flowers this can have a hint of Asteraceae about it, but look carefully there is no receptacle, no phyllaries and the flowers are like small pea-flowers (it’s the Pea family afterall!) and nothing like the ray and tube florets of Asteraceae!

So, in the end familiarity breeds Asteraceous content, and only by looking at a wide range of species can you be sure to be able to spot a decent Asteraceae a campus mile off!