On 11/06/2017 10:04, Nick Maclaren wrote:
In article ,
Clive George wrote:

Yes, exactly. So my question is, how long does the leaf have to be
around before it has generated enough energy to replace that stored
energy which was used to make it?

I have thought of that question in the past, and occasionally tried
looking up, but have found nothing useful.
I'd hazard a guess at less than a few days at least for those plants
like orchids and lithops whose seeds are little more than dust. The
plant has few reserves to fall back on after growing a root so the first
true leaves have to more than break even if the seedling is to survive.

They also add to the mass of the plant. I'd suggest as a ballpark number
the plant has broken even when the dry mass of the plant exceeds that of
the seed. Or for a ballpark with a live plant 5x the mass of a dry seed.

--
Regards,
Martin Brown