Post-floral whateries?

It was "that" hot day in July when the Purple Vetch seeds "pop" from their seed cases. As I stood trying to get a glimpse of it in action I became aware of two or three ants hanging around at every leaf node along the stems of the plant. What were they doing?

A busy bee on a Vetch flower

Whatever it was, they were all on the same mission and all seemed to have a bit of a thing for a nice green leaf node.

Now, I could narrow the behaviour down to something to do with reproduction, survival/defence or food source, which you can do with pretty much any living thing, apart from humans who seem to be constantly distracted by many meaningful (or not so meaningful) tasks. So I was heading for the food source idea.

I know that ants like to drink sweet stuff - who hasn't found one in a glass of fizz or squash at some point? 

And I also know that plants produce sweet nectar, but surely that's just from the flowers...

Well, it seems not. After a bit of research I found out that it's all to do with "Extra (or "Post") Floral Nectaries" (EFN) - a phenomenon which has barely been researched, especially here in the UK. 

It turns out that some plants produce nectar from their leaf nodes & other areas as well as their flowers, but why? 

Well again, it's either reproduction, survival or food source & from the plants point of view this is all about survival/defence.

A Purple Vetch wants to successfully reproduce & to do this it has to protect it's seeds from danger until it's pods are ready to "pop", at which point it has accomplished it's task. 

Drunk & Disorderly Ants

The plant & the ant have evolved a symbiotic relationship. 

There are many plant pests who would happily munch on a Vetch seed, but much like ants defend aphids in return for a sweet treat, in this case the ant is defending the plant & it's seeds in return for some of the plant's nectar - isn't that incredible! 

So, why don't ants just help themselves to the nectar directly from the flowers? Apparently, they are physically prevented from taking floral nectar by the dense stamens - yet another clever trick!