BOTANY 1: The world’s commonest green organisms

And they are not what you possibly think they are!

David Beeson, mid-May 2021

Biology is currently dominated by the FIVE-KINGDOM concept of organism diversity: plants, animals, fungi, protista and those organisms without a nucleus, such as bacteria – the prokaryotes. Generally, plants, animals and fungi are mostly easy to recognise. Protista contains those nucleated organisms which do not easily fit into those groups. The ALGAE are usually included in the PROTISTA KINGDOM because many of them bear little relationship with ferns or conifers or flowering plants, yet are mostly photosynthetic. And algae occur in huge numbers everywhere (billions on a single oak tree trunk) and I consider them the commonest green organisms.

Hint: Do you have your own microscope? I bought my ex-university Kyowa microscope via eBay for £100. New machines are available, but mine was great value, which I cleaned up and bought some slides, coverslips and stains to allow me to make my own slides. Commercial slides also available.

How about a university-level botany textbook? Mine? Second hand from eBay! Generally, botany textbooks do not go out of date. (Except the biochemistry!)

If you scrape a surface sample of most trees onto a microscope slide, add a drop of water and lower (from one side to avoid all the air bubbles) a coverslip you’ll spot thousands of spherical Pleuroccocus cells – a single-celled green alga.

Go to the seaside and look into a rock pool. The red, green or brown seaweeds are algae.

Look into a pond and the slimy threads are algae – probably Spirogyra or Zygnema. A drop of that same pondwater will be full of small algae.

Algae are everywhere, and in huge numbers because many are single-celled.

Divisions of algae: 1. Euglenophyta. Some are so mobile that zoologists claim them as their own! 2. Dinoflagellates. 3.Diatoms. 4. Chlorophyta – the green algae which I encounter most. 5. Brown algae of the rocky coasts. 6. Red algae.

54 Phytoplankton diatoms ideas | diatom, microscopic photography,  microscopic images
Diatoms. They come in beautiful designs.
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Fucus – a sea brown alga

Algae are organisms with nucleate cells, they photosynthesise and have a very different reproductive system to the more-typical plants, with some mobile sex cells (gametes).

I will only cover two alga types here: Euglenoids (Euglenophyta) and Chlorophyta, although any investigation of water will yield diatoms.


What a mixture! Some have chloroplasts, some never. Most are unicellular, yet a few form small multicellular organisms. Many can ‘eat’ food, while others absorb food through their outer layers. Even the photosynthetic species grow faster if they can engulf organic materials. They swim around using flagella (Whip-like structures).

I find them in green ponds, and most warm weather pond samples will yield some. You’ll spot them scooting around, and it will challenge your skills to work out their structure unless you kill them or buy a prepared slide.


See your own Euglena species

Collect some pondweed. Leave in a jar with pond water in the dark. A brown scum will develop on the water surface. This will contain Eugena species and amoeboids.

  1. Take a drop onto a clean slide. Add a coverslip, avoiding air bubbles by adding at an angle.
How to Mount a Microscope Slide - Rs' Science
  1. Always start with the lowest microscope power, adjust diaphragm, lighting and view of the slide for optimum viewing. Then work up through the higher powers.

You can find more details and videos on the Internet.


I am surrounded by chlorophytes! My garden pond is free of fish but full of green algae – Spirogyra, Zygnema and Chara, which are the easily visible types, and there are myriads of single-celled types, plus some beautiful multicellular, hollow balls of flagellate algae.

Transgender Algae Show How Males and Females Came to Be – National  Geographic Society Newsroom
Volvox – a colonial alga

My trees have green bark which is covered in green single-celled algae. Even the soil surface has algae on it. Plenty to investigate.

Algae from a tree trunk: Pleurococcus. Despite algae being water-loving, this one survives living in the open air, although it prefers the cooler, damper north-facing sides of the trunk. Each cell has an especially thick cell wall to limit water loss. It uses a carbohydrate-rich slime to adhere it to the tree. Bacteria now attack that exudate and a biofilm develops – an ecosystem in its own right.


This is one of many types of filamentous algae you will find. The cells above show the spiral chloroplast and cell walls. An investigation will soon allow you to spot the nucleus and the starch granules. But the reproduction is really interesting, and I’ll not spoil it by describing. Do look for yourself and you’ll see conjugation tubes and male and female gametes.

The best algae to seek is Chara. I used it with students because its cells are huge and you can see cyclosis – the flowing of cytoplasm within the cells.

The algae are a diverse group of organisms, and much neglected by citizen scientists. Yet they have a wide distribution, an interesting sex life and are easily studied. So, grab a good microscope and set yourself up for a winter project.

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