Annuals 101

Recently, Nathan Wong, Sera Cutler and I were looking at patterns of plant diversity in Yellow Gum woodlands in the Wimmera (Yellow Gum woodland species diversity). Two things struck us - (i) there were up to 104 species in an area only 32 m by 32 m (making them some of the most diverse plant communities in the world at this spatial scale) and (ii) annual plants contributed, on average, about 40% of the plant species were saw at most woodlands.

The 'inch flora" - hard to see!

So, why is the second observation of note? It would appear to me that the important contribution made by annuals to biodiversity in eucalypt woodlands in the 300-500 mm rainfall belt has been largely overlooked. This probably has something to do with their small size. There isn't much information on the biology and ecology of annuals from southern Australia at all. We know that there are many annuals in the Asteraceae, Centrolepidaceae, Crassulaceae and Portulacaceae, but we don't really know much else.

So, in this contribution, I thought I'd outline some of the basics about annual plant morphology and seed biology. Teri O'Brien, a cheerful retired plant physiologist who helps out in my lab, has started to unravel the secrets of the "inch flora" and I'm grateful to him for this information.



Siloxerus: an example of a 'strict' annual
(Photo: Pete Green)



1. Height: annual plants can be characterised as either 'strict' small plants (always <30 mm regardless of environmental conditions) or 'variable'. In the later group, height seems to depend on resource availability. In good rainfall years, for instance, heights of 100 mm or more are attained.



Gnephosis: an example of an 'erect' habit
(Photo: Pete Green)

2. Morphology: annuals develop either (i) a single shoot which bears just one terminal inflorescence (‘erect’ habit), or (ii) one or more additional shoots are produced from lateral buds on the primary axis (‘classic’ habit).  These lateral shoots appear to grow diageotropically and then curve upwards, bearing either inflorescences or further shoots that, in turn, bear inflorescences in a more or less erect position.



Evidence of myxospermy (foreground) compared
to a species which is not myxospermous

3. Seed biology: some annuals have myxospermous seeds. Here, the seed coat contains a mass of dry mucilage that hydrates and swells rapidly on contact with water, in some cases in as little as a few minutes. A number of theories have been suggested about its significance but I'm inclined to the idea that it assists the seed to attach to the soil, giving the radicle something to push against as it starts to elongate, minimizing the risk of pushing the root hair zone away from the soil surface.  



4. Seed germination:  the common seasonal pattern of high summer temperatures and a variable period of low rainfall combine to render the seed of many annual species initially dormant upon dispersal. As the summer progresses, the high temperatures that seeds experience at (or near) the soil surface appear likely to slowly reduce the dormancy of the embryo (this is called dry after-ripening) which eventually responds to the combination of cooler conditions and moisture by rapidly germinating. 

So, it's clear that annuals are a fascinating group of plants which warrant much better ecological understanding. Further quantitative investigation of the seed biology, seed germination requirements and developmental anatomy are clearly necessary.  Next time you head into your local bush, take the time to search for these much overlooked group of plants and marvel at their beauty.