The short answer is: phylogenetic constraints.

For instance, in the asteracids what we can assume happened is that the mutations required for production of latex occurred in the common ancestor of the Cichoroideae after divergence from the other tribes. Latex is presumably advantageous, so most (perhaps all? I don't know offhand) cichoroids maintain this character, but the other asters just haven't had the right mutations come along. If this is the case, the other characters that differentiate the cichoroids from the other asteracids derive from the same phylogeny but are otherwise completely unrelated.

Patrick Alexander

Could there be an evolutionary advantage? Latex can reportedly have some insecticidal properties. Would latex flow more easily from one kind of petal than the other? It would not be a biological advantage to kill a pollinator.

This is a really interesting subject. I'm glad you brought it up. I will be doing research on it in Google now.

Thanks,Patrick and Eibren!

I've put this question here and two botany forums in China.I'd like to make a short summary of our discussions in China as to better exchange our ideas.

1. latexes are generated in laticifers and seem to have sth to do with the latex cell,which I'm quite unfamiliar with.(I'm a pure math major :) )But I think I'll check a plant physiology book several days later(I'm now preparing for an important test.)

2.Eibren,as for your comment,I've also noticed sth about latex and insects.I discovered here in our campus,on the species Hemistepta Lyrata (Lyrate Hemistepta your English name?) and Cynanchum Chinense (China mosquitotrap your English name?),which are both abundant with latex,there are a lot of aphids!!Maybe these two species are not in your land,but I think that maybe there are similar species are species within the same genus or even family that have the same attractiveness to aphids.Of course,there are also aphids on some species where no latex in present.

I searched the web and see sth on this topic,it says that some plants develop latex as a defence which would stuck the insect's mouth as to stop its feeding.But insects have also "devised" methods,they would either use some physiological or biochemical mechanism to degrade the toxic substance or they will feed themselves selectively as they choose places where there are no latex(maybe the leaf blade?).And it is said that some insects might cut the laticifer or dig some "whole" to block the flow of the latex!!

And as for your suggestion about the petal,Eibren,I'm quite uncertain,are there latexes in petals??But it's quite an provoking suggestions,I would do more observations later.

It's a bit long and tedious now,I think I should start a new post.
:)

Hui Gao

Aren't milkweeds (Asclepias spp-A. incarnata, A. tubererosa....) an example of coevolution and evolutionary advantage? No latex in the petals but plenty of latex in the leaves and stems. Monarch butterfly cats eat the leaves, the latex is utilized as a defense against pretators that eat them. Also, the latex prevents other predators on this plant.

Indeed interesting .. we have several species of Euphorbia here that contain layex and are often inundated by ants ??

I oftened wondered if latex in leves effected water relations of leaves ?? .. like a radiator coolant.

Good Day ...

inundated by ants ??

I think that's maybe because ants are there herding some aphids.You can take a closer look.

I happened to see a very interesting article on the interaction between insects and some plants,which have latex!It's indeed an interesting reading.Hope you'll enjoy that.

Hui Gao ^-^

Here is a link that might be useful: the article

I believe that you will find the answer to your query if you can find a copy of PLANT PHYSIOLOGY, 2 ed, by Taiz and Zeiger,published in 1998, page 357. The library call letters are QK 711.2, T35.
Good luck in your quest!

taxonomist,
Thank you!
We've got a 3rd edition in our library,and I will check that out!

Hui Gao