What is a plant? (part 1)

Plants are green aren't they? And they grow, and they don't walk about.  Well mostly...  As well as the green plants, Glaucophyta and Rhodophyta (red algae) are usually regarded as plants, but I'm going to leave them aside for the rest of this posting and focus on the green plants.

In the introduction to my PhD thesis I wrote that 'Green plants are characterised as containing chlorophyll a and b, storing photosynthetic products such as starch inside chloroplasts, and having cell walls made of cellulose (McCourt et al., 1996)'. In retrospect, this is a reasonable-ish definition, but is limited to describing shared morphological features, and doesn't necessarily speak to aspects of phylogeny and most recent common ancestry.  The word 'synapomorphy' might have been more specific, implying that the features were inherited from a common ancestor.  The Tree of Life web project (http://www.tolweb.org/green_plants) contains a similar morphological definition and also circumscribes the green plants as 'all organisms commonly known as green algae and land plants, including liverworts, mosses, ferns and other nonseed plants, and seed plants'. That page also has a great list of links and references. Now we know what we're talking about. Kind of.

Palaeos.org (http://www.palaeos.org/) takes quite a nice look at what a plant is, and makes an attempt at situating plants in time as well as space, explicitly delimiting the group of organisms (Chlorobionta) which we can call green plants, and which we think has a single common ancestor. They also observe that this involves the use of the taxonomic hypothesis of common ancestry, which, as a hypothesis, may well turn out to be incorrect, despite strongly supported phylogenies.

So now, as well as talking about what a plant is made of, and what kinds of different shapes and sizes they come in, we raise the question of time. When did the first common ancestor of all the things we call plants arise, and also, what (and when) was the first thing that we might recognise as a plant (the two might possibly be different). That can be another story for another day.

For now, I'd just like to share with you my favourite green algae, Aegagropila linnaei, or Marimo, as the Japanese know them, or Kuluskitur in Icelandic. They used to be called Cladophora aegagropila, when people thought they were closely related to the Cladophora seaweeds, but molecular evidence said otherwise (Hanyuda et al., 2002). Marimo are different from many plants, as they live under water (like lots of the green algae though). In fact, they live in only a few lakes in the far northern hemisphere, in cold, shallowish, brackish waters. Also, unlike lots of plants they are not anchored to a surface, they are free-living.

They grow in the form of green balls, up to several inches across, and either roll around on the bottom of the lake, or sometimes, photosynthesise and generate bubbles of oxygen which allow them to float up to the surface. Unlike their close relatives, the Cladophora seaweeds, they express chitin as part of their cell walls, so are quite 'crispy'. Also, unlike many plants, they seem able to shut down photosynthesis in the absence of light for long periods, then rapidly reform the chloroplasts when light is available again (Yoshida et al., 1998).

They are a protected species in both Japan and Iceland, but do pop up on ebay from time to time. I'm not sure of the original source of the ones for sale. In the 1990s there was a journal called 'Marimo Research', but it seems to have disappeared without a trace, and I haven't been able to get hold of a copy. If anyone knows where to find it, I'd be very pleased to know.

My pet marimo live in this jar, in ordinary mineral water from the supermarket. The water hasn't been changed for a year or so, and is still clear, and they are still growing happily, but I have seen recommendations that you change the water monthly, and massage the marimo to help them stay clean. They are somewhat sacred beings with many myths and legends surrounding them. A little bit fell off one, and I looked at it under my old russian microscope, you can see the chloroplasts quite clearly. This second pic was taken with a digital camera through the eyepiece.

To be continued...

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Hanyuda T, Wakana I, Arai S, Miyaji K, Watano Y,  Ueda K.  2002. Phylogenetic relationships within Cladophorales (Ulvophyceae, Chlorophyta) inferred from 18S rRNA gene sequences with special reference to Aegagropila linnaei. Journal of Phycology 38: 564-571.

McCourt RM, Chapman RL, Buchheim M, Mishler BD. 1996. Green plants. Version 01 January 1996 (under construction). http://tolweb.org/Green_plants/2382/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/.

Yoshida T, Horiguchi T, Nagao M, Wakana I, Yokohama Y. 1998. Ultrastructural study of chloroplasts of inner layer cells of a spherical aggregation of “Marimo” (Chlorophyta) and structural changes seen in organelles after exposing to light. Marimo Research 7: 1-13.