Question

I am doing a project. I have to determine which of the following plants are monocots and which of them are dicots. The plants are Pansies, Hydrangeas and Sunflowers.

Answer 1

Hey there,

If you're looking for just the answer, you could throw those names over into Wikipedia - it gives the scientific classification for you. They look to be all eudicots.

Answer 2

What is your project specifically asking you to do though?

Answer 3

I am looking for an explanation. My text says one way, but online says the other.

Answer 4

Ah okay thanks for clarifying. For what example do your sources clash?

Anyways, if you're going by traditional classification schemes, which are based on morphology, you'd have to observe each one and see which characteristics apply. There's a good list right here: http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html
For example, if you take a closeup look at pansies and hydrangeas, you'll see that each follows the 4 or 5 petal rule - indicative of dicots.

The confusion may arise from the fact that biologists no longer primarily go by morphological standards when classifying organisms. The usage of molecular phylogeny has offered a better way of defining relationships through evolution. The term dicots is now obsolete, I believe, and is replaced by the monophyletic group: eudicots. So, one source may call an organism a dicot based on older, morphological standards, but a newer one may place that same organism in a different group based on newer data.

Apologies for not being able to offer more help!

Answer 5

Here are some common monocot characteristics that you can usually see without needing to look at anything under the microscope:
- one seed leaf (if you're lucky enough to see the plant germinate)
- a number of petals/stamens/etc. that's a multiple of 3 or 6
- leaves with parallel nerves
- a crown that is not differentiated into petals and sepals
- GENERALLY no secondary growth, so with the exception of some tropical species, most of them stay reasonably small and don't grow into trees

Note: these are all common characteristics, but there are exceptions! The Spathiphyllum sitting next to me (from the family Araceae, a monocot family) has leaves that do not appear to have parallel nerves and its flowers are organised in a complex spadix (with no petals!), surrounded by a petal-like leaf (spathe) that opens up when the plant blooms. Here's a picture to show you what I mean: http://upload.wikimedia.org/wikipedia/commons/0/01/Spathiphyllum_cochlearispathum0.jpg (Mine is a different species, but it looks pretty much the same except for the colour.)

You should also be aware that typical "monocot" characteristics sometimes show up in dicots as well. Plants in the family Ranunculaceae (the buttercup family) don't have sepals, though some of them look like they do. But we can figure out that they're dicots because the petals are multiples of 5, the leaves don't have parallel nerves and they are also often divided (not common for monocots).

But... if you're looking at the average non-tropical monocot, there's a good chance that you'll be able to spot the traits that I mentioned.

Grasses, for example, are monocots. How can you tell? They have leaves with parallel veins and flowers with at least some parts that show up in multiples of three! Tulips are also monocots. How can you tell? Count the petals (typically 6, I think), count the stamens (again, typically 6), check if there are sepals (nope!), and look at the leaves (they have parallel nerves).

Oh, and to add to what @kma230 just said, it's true that "dicots" are not a monophyletic group. That really just means that the term "dicots" encompasses groups that aren't closely related but share certain morphological characteristics, kind of like "worms". So you can use it, but you should know that it's not a real taxon.

Monocots, however, ARE monophyletic. That means that if you look at the closest ancestor of the monocots, ALL of its descendants are monocots. What happened is that monocots evolved within the "dicots". Take a look at this graphic and look at where it says monocots: http://www.life.illinois.edu/ib/335/Monocots/monocotAPGII.jpg

The fork above that word is where the monocots begin. You'll see that they're all one cohesive group: if you were to snip through that one little line, you could pull every single monocot and not one single dicot out of the tree without snipping any other lines. Dicots, however, are everything else in that tree. So... Eudicots (which are monophyletic), Magnoliids, and several others branches. They don't share a common ancestor that is not ALSO the ancestor of the monocots.

Answer 6

Great answer, @Calliope !

Answer 7

Thank You @kma230 and @Calliope for clarifying. Many Thanks to you both!