Hello Luke,
First you will put cuttings (with their green leaf stems still attached) in that gallon sized zip lock plastic bag that you will put inside of an empty quart wide mouth plastic container.......see picture. You give those cutting air by changing the water very often......shaking too is good. Don't put too many cuttings in one single plastic bag because they will be using up oxygen while they are in that bag. And you do not want for them to "smother" and get rotten. Remember to keep that bag OPEN at the top. So change that water often and put as much air in it as possible. Use a fish aquarium air pump if you have one. After the stems fall off, then you put them individually in small mouth empty plastic soda bottles. Those cuttings won't fit in those small mouth bottles until the stems fall off.
Once you have your cuttings in the small mouth soda bottles, you can add air to these by putting your finger on the mouth opening and shaking as directed. As long as you keep oxygen from the air in the bag and oxygen from the air in the soda bottle......you WILL NOT have any problems with any rot or mold. If you do not add air....you WILL have problems. It is very very very simple to insure that you always have oxygen from the air in that water at all times.
Yes, those leaves that open up from a closed bud WILL start photosynthesis right away. That is because those cuttings are already "alive" and the internal pipeline is fully functional. Those leaves will absorb carbon dioxide "from the air" and produce oxygen and more food (energy reserves) for the cutting to use.
Now don't get confused with this.......inside of the bottle where the roots are growing, you can have photosynthesis coming from any algae that is growing on the wall inside of the soda bottle. A cutting needs more oxygen (in this example it will take oxygen "from the water") to combine with its internal energy reserves to make those new root cells that you will see growing. In that biochemical process of creating those "brand new roots" that cutting will make Carbon Dioxide gas "in the root area". In this case, the carbon dioxide that is made BY THE ROOTS will just dissolve "in the water".
Now algae loves to eat carbon dioxide that is dissolved in water. Carbon dioxide is food for algae. So sometimes you will see algae grow in your rooting bottles. That is not a problem and actually can be quite GOOD. That algae will use the carbon dioxide that those new roots are giving off and dissolving in the water bottles.....to use for itself. That process that the algae is using is called photosynthesis too.
But through photosynthesis, the algae is making food for ITSELF TO USE and to grow even more algae. What the algae does for "the cutting" itself is give it more oxygen. Which is GOOD for the cuttings too. So the algae will remove the carbon dioxide that the cutting makes when new roots are forming and in return the algae will produce more oxygen in the water that the cuttings need to produce even more "new" roots. This is a win win situation for both the algae and for the cutting whenever this does occur.
Don't worry at all about this algae thing. If you see it...fine. And if you don't see it....that is fine too. Just make sure to add air to your water and all will be just fine. Like I've said, this is very very very easy to do. However, there is some real science going on inside of those bags and inside of those bottles.....and I mention that for those who want to know WHY things happen or WHY they sometimes don't happen.
Dan
Semper Fi-cus