Very good! The graphics are beautiful, the controls handle well, the music suits the mood perfectly, and the whole affair feels very polished. At first, I thought the plant multiplying into several stalks was going to make the game too easy (as it seems like, whenever you crash, there'll always be one surviving stalk), but I found that when the area of stalks you're concentrating on crashes, you have to quickly shift focus to another area of stalks, which makes for an interesting challenge.
The one thing that feels missing is a sense of progression. In other words, it seems like the game is supposed to be about seeing how far you can go, but there's little indication of how far the player has come, nor any particular reward for getting further and further (at least, as far as I got- I got a few checkpoints in before it got too difficult, generated a new level, and found the difficulty about the same). As such, my only motivation to keep going (aside from the general quality of the game) was the implication in the description on this page that there was eventually a surface to be reached (though I have no idea how close I ever got to it).
I see a couple of other people suggested including a score counter- that's one way to do it, though I think they're assuming the game is endless. Assuming I'm right that the game has an end point (reaching the surface), then a similar method of tracking the player's progress could be a slider that slowly moves up as the player gets closer to the surface. Either method would be an indicator of how well the player is doing that would motivate them to keep going. However, I think the game's lack of GUI elements is something worth holding on to (it really enhances the mood), and that you could achieve this effect a bit more subtly. Perhaps the cave should change appearance as you get closer to the surface- more sunlight gets through, moss starts to appear, that sort of thing. Maybe you could also take advantage of the parallax effect by putting large, distant objects in the background as landmarks for the player to slowly move past, such as more cave openings, underwater lakes, or even ancient underground temples and stuff.
Also, a far more minor note: the instructions should mention that Esc is the pause button- I had to do a little hunting for it (I tried Enter and Space first).
At any rate, I really enjoyed this. Good work!