This is so awesome!
Though I dare say I knew from a similar technique the moment I started photography.
Images crated onto the surface of plants or using plant dyes are called Anthotypes. There's interesting stuff on the web. The most impressive work was made from Bin Danh, printed on various leaves.
There's three main concerns to the technique:
1. The exposure time is long. Most people get hours on sunlight for getting an image on paper, more for leaves.
2. For using this process you need a positive/ transparency. Since the sunlight destroys or "bleaches" the colour, the process is positive - positive.
3. The image cannot be fixed and therefore is ephemeral.
To me, these weren't that much of a concern so I tried it. I used chlorophyll and turmeric extract for the dyes. The first thing I found was that exposure time for me wasn't as bad as internet told me. Prints made from chlorophyll could be made on 10 minutes in full sun, I even got to make prints using a 15w cfl lamp very close to the printing frame. Second, after thinking a little bit I thought that negatives could be used, as long as the plant leaf wasn't cut from the plant while printing. This is because, like on the algae-on-petri-dish method described above plant leaf cells get green where they find light, if they don't find it, they turn discolored, usually white or brown. So if you just keep the plant leaf attached to the plant with a negative on top the areas that are not exposed are going to lighten, bingo: a positive print. The opposite happens when the leaf is cut.
Lastly, I wanted to say that although the wish of having a permanent image is very strong, I consider that sometimes it's okay for it to fade, specially when talking about anthotypes. The dyes are strong enough to stand a wide variety of exposure to light. It won't fade quickly. You can show the prints on full daylight without getting them ruined. It's the prolonged sunlight that makes a difference here. I save my chlorophyll prints inside a book in my shelf, and so far two years have passed and they're looking the same. The turmeric ones are complicated because the curcumin is unstable and after some time the image disappears, it gets blurry and the overall contrast turns into a mild orange. But, hey, then you can print again. I find the ephemeral property of anthotypes to be quite encouraging to keep making prints. And they in some way resemble life itself: we're all going to die somehow and everything has an end. It's okay for it to be that way. Life moves on.
Sorry for this.
