This solution uses a combination of Photoshop and Illustrator.
- In Photoshop, use the Clone Stamp tool, with a large soft edged brush, to clone pieces of the leaf until you have a kind of random leaf pattern texture. You can set the target for the Clone Stamp by doing Alt/Option+click. You will have to do this several times as you continue cloning.
This example is speeded up, but should only take a few minutes
Copy and paste this into Illustrator.
Type some text on top of it.
- Select both text and image and do Object > Clipping Mask > Make
This is very will possible, but it's a bit tricky to it right. This is because this method requires that the text shape is a single, closed, non-grouped, compounded shape, and that's sometimes difficult to achieve.
- Select your text;
- Text > Create Outlines (Shift+Ctrl/Cmd+O)
- Object > Ungroup, possibly a couple times;
- With everything still selected, use the 'Unite' option in the Pathfinder palette;
- Position your pixel image behind the text in such a way that the texture underlays the text everywhere (and doesn't run off the leaf, for example);
- Select both your (single, closed, non-grouped) text shape and the pixel image;
- Object > Compound Path > Make (or Ctrl/Cmd+8.
If it doesn't work, you might try to Object > Expand, ungroup and Unite (Pathfinder) a few more times until you have your single shape.
This effect is way easier to achieve in Photoshop, as in a few clicks. If you are not extremely strictly limited to Illustrator, I'd go for the easier solution myself.
Finally: do note that the kerning (letter spacing) between the A and the Y is very much out of sync with the rest of the text, they should be much, much closer. The A and the D could be a bit closer as well, considering lots of the other letters actually run into ech other.
You have got suggestions to clip pieces of a leaf with letter outlines. I tried it, too. Outlined and ungrouped letters are used one by one as a clipping mask for a copy of a leaf. The leaf is actually traced to 8 colors (+ignore white) to make the texture sharper.
It's difficult to see them as leaves because surface details do not define leaf, it's the outline. These are letters which could as well be made of stone.
Using an image of a mass of leaves do not make it better:
I suggest you build the text of leaves. Have several of them to avoid too much repeating. The next is an example.
A piece of distorted text is covered by copies of two different versions of a leaf. They are separated from a photo in Photoshop and placed to Illustrator
The text would look better as 100% leaves. The underlying vector text should be unnecessary if you have say 4 different leaf photos.
Small leaves could be sprayed. The letters could be drawn if one has a brush which scatters leaves.
Dragging leaves manually from a bunch of randomly sized and rotated versions is also possible. The next image contains quite small copies of one leaf.
There was AB (Arial Bold) as a guide in the bottom.
Drop shadow here and there clearly improves the result. Without it the leaves look like they were glued together. Actually it was used also in the former example (=the LEAF) for the same reason.
One idea is still untested: Create a leaf like shape with surface, edge and veins which has got its main outline from a letter. Think it as taken from a tree where the leafs are letters. I have still no idea how to make it at least partially automatically. A competent painter of course would draw it and a competent math programmer could declare it as rules which are applied to the needed letters.