For today's species spotlight I have chosen another very common grass - if you live in the UK it is almost certain that you could step out of your front door right now and find it growing out from a crack in the pavement or on a wall. It is Annual Meadow-grass (Poa annua) - a very widespread species which is thought by some to have possibly originated as a hybrid between two other species in the genus Poa, although I don't think that view is shared by all authorities. In North America, where it is an introduced species, it is known as Annual Blue-grass and is a popular choice by golf-course green-keepers for putting greens.
I have chosen it today because it is common and easily recognised and also because it is a good species for illustrating some of the terminology that you will come across if you ever try to identify grasses using a book. I am going to focus on leaf structure today, and leave the flowers, fruits etc. for another species and another day.
The three main terms that you will hear in relation to grass leaves are 'leaf-blade', 'leaf-sheath', and 'ligule'. The first of these is the most obvious - you will often have heard people talking about blades of grass, although you may have thought it meant the whole plant, or the stem. Actually it is the usually broad flat bit which looks like it is the whole leaf.
The second term - leaf sheath - refers to the lower half of the leaf, which, in many species, is as long as, or longer than, the blade itself. If you take a grass stem and gently pull on the leaf blade while holding the stem in your other hand, you will usually find that the blade pulls away, revealing a long tube of tissue connected to it, which was, until you pulled it away, wrapped around the stem. This is the leaf sheath. I say 'usually' because some species have the sheaths closed, so that they won't easily pull away from the stem without tearing.
Now for that strange word 'ligule' - it comes from the Latin word 'ligula', meaning strap, which itself comes from another Latin word 'lingua' meaning tongue. In grasses it refers to a little flap of tissue, usually thin and colourless, which goes up along the stem from the top of the leaf sheath. It can most easily be seen when you pull the blade away from the stem as described above - you should be able to see it on the second and third pictures below. The shape and size of the ligule varies a lot between species and is often important in determining the correct identification. In Poa annua it is usually between 1 and 3mm long and smooth edged.
Another phrase that is used to describe the leaves of certain grasses - the Meadow-grasses (Poa) and the Sweet-grasses (Glyceria) being two groups that come to mind - is 'boat-shaped tips'. You might be able to see it on the photos below (although it is not as obvious as I had hoped) - it is where the central vein of the leaf is longer than the two sides, causing them to pinch together and push the bottom out like the keel of a boat, making the tip of the leaf-blade look like the bows of a rowing boat. In Annual Meadow-grass it is particularly obvious, so much so in fact that it is sometimes described as 'hooded' rather than 'boat-shaped'.
Finally, I will mention one feature which is often found in Poa annua and which can be seen on the third picture below - that is the concertina-like wrinkles, or crimps, in the leaf blades. This is not always present and its absence should not be used to rule out Annual Meadow-grass. Also they may disappear as the plant gets older.
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