Veg plant ID: how to tell if rosette plants have opposite or alternate leaves?

Dr M is used to a lot of questions from his students on the MSc Plant Diversity University of Reading!

And once they started using the veg key then there’s been a lot more questions! But one of the most frequent is “Dr M, with a rosette plant how can we tell if the leaves are opposite or alternate?”

With a normal plant, i.e. one with leafy stems, it’s easy, opposite leaves are opposite and alternative leaves, well they are alternate! But with rosettes, where the leaves are all squished up together, well it’s not so easy, hence this frequent questioning.

And it’s an important question because with rosette plantsif you can’t answer this question you can’t progress from page 1 of the veg key!

Well, Dr M was walking back to his office after lunch recently with his colleague Alastair Culham, and who should they come across but one of their MSc students who had a question. You guessed it!  Rosette plants, opposite or alternate leaves, Dr M, tell me how I can tell?

As it happened looking down at the ground there were rosette plants a plenty in the open vegetation at the base of a concrete bollard so we were ideally placed to offer an explanation in the field!

OK, you have some herbaceous rosette plants (the Danish plant ecologist Raunkiaer called them hemicryptophytes) like those labelled in this picture and you want to know if the leaves are opposite or alternate. How do you do it?

Well, rosette plants are plants which have a stage, often over winter, when the internodes are all extremely short and all the leaves are stacked on top of each other, and because leaves are arranged around the axis of the plant then they look like a rosette  – just like you get when you win a horse race or that politicians use at election time to show their true colours. Well we need to examine these plant rosettes and check their true colours – opposite leaves or alternate?

We have just two choices so it can’t be that difficult!

One answer “wait and see”. Many of these rosette plants will develop leafy shoots in the spring and summer and then it’s easy. But not all of them will do this, Dandelions and Daisies for example are out and out rosette plant and they never have leafy shoots, so waiting and seeing is not always an option!

So we really DO need to be able to tell just by looking at the rosette. And of course we can, and it turns out it’s not rocket science, though you can call it eXtreme botany if you like, but only level one!

The rosette has opposite leaves when pairs of leaves are juxtaposed, and alternate when the leaves alternate left then right. Actually in reality leaves often spiral and this makes the neatest rosette, but to simplify we just ask opposite or alternate, maybe thats part of the confusion, maybe if we said alternate can also mean spiral it would be clearer?

If the leaves are opposite then when you look carefully at the rosette you will be able to find pairs of leaves of exactly the same length. Start in the middle, with the smallest leaves, (the youngest ones) and you should see a pair of equal sized leaves. Then moving outward to the next leaves you should see another pair of leaves both the same size but slightly larger than the first pair. And this will continue as you check more leaves. You will often find that the leaves are opposite and decussate, that is the second pair of leaves is arranged at right angles to the first, and the third pair at right angles to the second and so on.  This arrangement makes a rather tidy, paired leaf rosette which, once you get your eye in is very obvious. Typical families with rosettes of opposite leaves are some Caryophyllaceae (e.g. the Mouse-ear’s, Cerastium species) and Onagraceae (the Willowherbs).

If the leaves are alternate (or spiral) then each leaf will be a different size with NO pairs obvious. If, once again, you start with the smallest leaf (the youngest one, in the centre of the rosette) and compare it with the one next to it (the second youngest) you should see the youngest leaf is the smallest and the second youngest a bit bigger, the third is bigger still and so on. There will be NO clear pairing of leaf sizes and the rosette will look like a real rosette with leaves arranged in an overlapping spiral.

Is that clear?  Well test yourself, check out the images above, all taken from that little patch of vegetation by the bollards, and see if you can decide opposite or alternate?

Check the answers here!