How do plants and trees work?

Okay, there's currently a thread in feature suggestions about giving users the ability to create their own plants/trees that look as good as the Linden ones. By all appearances the technology is there, but we don't have a way to use it.
The thread in question is here.

However, my aim isn't to bring particular attention to feature request as such. But more curiosity on my part. What I'm wondering is if we could get any details on how the plants are done in SL?

My thinking is that they are either:

1. Included with the game and take on their own format. They are thus essentially pre-cached so can be loaded quickly (and scaled down as performance/distance demands). Possibly in some kind of special geometry format to specify flexible parts
   
2. A different interpretation of a regular object. Perhaps a flag in the object that says "This is a plant" and causes SL to interpret its data differently, the data being various parameters relevant to plants and trees. Alternatively a list of values like particle systems (as per Argent Stonecutter's suggestion).
   

Are either of these correct? It would help in determining how exactly the feature would be suggested and what an ideal solution would be.
Linden plants and trees, while good looking are limited, both in the range available and the use they can be put to, as they can't be added to linked sets, or modified, or put into object inventory for rezzing and so-on.

Thanks for any info we can get

I'm not sure, so I've asked one of our artists to help you out. You should be hearing from Eric Linden soon!

As you all know, I created the assets for the current “Linden” foliage in SL a while ago, and I’ll try to address some of the comments in this thread...

Our current trees and plants are rooted in the code-base in a manner consistent with L-system technology, and are streamed to your machine from the servers in the same manner as the terrain. Mapping and texture information in the code are very specific, requiring a common texture design/layout. Further, there is a limit on the resolution of these maps for the sole purpose of minimizing their impact on frame rates. Alpha is handled no differently than any alpha-textured prim in-world. All of the trees and plants respond to wind in exactly the same manner, with no modifiable parameters in the code. The cost of a Linden-created tree is similar to that of a single primitive. So, when you make your own trees – yes; they look great, but they are costly in terms of rendering (alpha, large textures, multiple textures), and in terms of higher prim count – PLUS – they won’t render as quickly as Linden-created trees.

All this said, we’ve been exploring options in other technologies, and the development of these things must take a back seat to security and stability. It is clear there is much to be gained from revamping our handling of foliage; we’d see dramatic increases in the visual quality of our environment, Residents would have far greater control over these elements, and we’d probably see performance increases as well. I’ve been trying to drive some development of new technology, but in the face security and stability issues (being the highest priority tasks), new foliage development must take a back seat.

Believe me; I want new trees, plants, atmosphere, media capabilities, procedural modeling and mapping, and on – and on – as much as anyone. Second Life involves a lot of code, and we are working hard to provide the most compelling and functional experience we can. There’s more to come from Linden Lab – stronger, faster, more beautiful….so please hang in there!

Eric