figuring out inflorescences? need help

17 years ago

Hi,

I am working an a computerized plant guide that translates

a key into laymens language. I have a biology background

but its in frogs not plants.

My confusion is this:

As I understand it there are inflorescences (non-simple ones) which are racemose (indeterminant so the older flowers usually appear at the base of the inflorescense) and those that are cymose (determinant so the older flowers appear at the top of the inflorescence). So far so good.

The confusion comes when I look at scorpoid and helicoid cymes. It would appear that the oldest flowers are at the base of the inflorescence yet these are classified as cymose. Of the examples I have looked at Phacelia stands out as appearing this way (its helicoid).

The only way it makes sense is if the first and oldest flower is considered the terminal one and all the others are coming off of one branch below it. It sure doesn't look this way though.

I looked at all the explanations on the web but there seems to be considerable confusion about the usage of the term

cyme (is it sometimes used casually to just describe the shape of the inflorescense??).

I apparently need to acquire a good botany text but I

haven't yet so please help me understand this if you can & recomend a good text if you know of one.

Thanks,

Kate

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Comments (5)

rhizo_1 (North AL) zone 7
17 years ago
There really isn't any confusion regarding the term 'cyme', but there are many interpretations (expressions) of this inflorescence. I would say that the one single unifying characteristic between all cymose inflorescences is that they are determinate. In some cases, that might not be observable but has to be learned. You've mentioned a couple of good examples of this.

Here is a link that might be useful: Try this one on for size
The_Mohave__Kid
17 years ago
A useful book is "Plant Identification Terminology" by James G Harris and Melinda Woolf Harris.

Good Day ...
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kate_rose
Original Author
17 years ago
Rhizo,

If I understand what you said there can be a cyme, helicoid or otherwise, which is completely morphologically indistinguishable from a raceme because there are no characters to differentiate the two (at least not unless you look on a developmental or cellular level). I mean none that could be seen with a hand lens. I read in the link you provided that in the helicoid cyme that the 2ndary floral axis arises from between the bracts of the "terminal" flower and continues in this pattern in some species. Is this a consistent character that could be used to distinguish the 2 or is this just a sort of exception to the rule that must be learned as you mentioned.

Mohave, I hope you had a good day too. Thank you for the reference.

Thank you both for the help.

K-

P.S. sorry about the mix up in terminology (red face)
for some reason my brain was reading determi"nant" instead of determi"nate"
kate_rose
Original Author
17 years ago
If anyone has input on the above I would really appreciate it.

Thanks,

Kate
paalexan
17 years ago
"The only way it makes sense is if the first and oldest flower is considered the terminal one and all the others are coming off of one branch below it."

That's pretty much what's going on. You've got a stem that terminates in a flower; the first axillary bud below it then produces a new stem that contains one axillary bud and a terminal flower; the axillary bud of that stem produces yet another stem that contains another axillary bud & terminal flower; & so on & so forth.

You could get an apparently identical inflorescence via a secund, coiled raceme; but in this case, flowers would be arising from the axils of the bracts, whereas in a cyme you have new stems arising from the axils of the bracts.

That distinction falls apart, however, if your inflorescence has no bracts! & then we are left with either studies of development, or with inference; for instance, if you see a coiled inflorescence in Phacelia that can't be definitely identified as a cyme or a raceme, but you know that the closely-related genus Nama has open cymose inflorescences, it's a good bet that the coiled inflorescence in Phacelia is also cymose.

Patrick Alexander