On 10/06/2017 18:58, Chris Hogg wrote:
On Sat, 10 Jun 2017 18:28:29 +0100, Clive George
wrote:

It's not just seeds though is it? Every year trees poke out new leaves,
and the energy/material to create those comes from somewhere.
Photosynthesis will at some point create enough new stuff
(energy/material) to make that worthwhile, but eg a leaf weighing 1g
will need to photosynthesise that much to do so.

Think about a potato - the tuber is basically an energy store, and one
day it sprouts and starts using that energy to create those sprouts and
leaves. It is investing that, in the hope that the sprouts and hence
leaves etc will return that energy and then some. My question could be
phrased as how long does the potato take to break even.

So I suppose another question is how much stuff does a leaf generate in
a day?
Do leaves only grow as fast as they photosynthesise, ie only during the
day, or can they used stored food from the parent plant to grow? I'd
expect the latter.

(and yes, I know about photosynthesis, that's kind of the background to
the question)

Hmm...OK. But the energy to produce new leaves in Spring comes from
sugars and other carbohydrates stored in the twigs, branches etc from
the previous year's leaves. The same with potatoes: they store energy
from the previous year as carbohydrate; it's what a potato is. In
round figures, a potato contains about 80 Calories of carbohydrate per
100g, i.e. about 20g of carbohydrate per 100g, the rest being water*.
Those 20g fuel the early growth until the stalk and leaves get above
the surface and start the photosynthesising process all over again.
While it probably isn't 100% efficient (few chemical reactions are,
and I don't know how inefficient it is, which is probably the nub of
your question), it's basically just one form of carbohydrate (the
potato) being converted to another form (the stem and leaves). Its
much the same with leaves of trees, I imagine.

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?