For the cave plants, my analysis actually started with the cavern animals. Chemosynthesis or nutrients washed down from the surface will not be anywhere near enough to support dralthas, let alone jabberers. In order to support the observed ecosystem, local autotrophs are necessary.
Caverns obviously lack sunlight, but this is a world where undead and various inorganic creatures exist (the general term seems to be "element men"), and presumably plants could tap into this energy source as well. I was not the only one with this idea; there is an older forum thread on the subject (in the "general discussion" section), under the name "
xenosynthesis".
Plump helmet and Dimple cup are explicily declared to be mushrooms, but I assume that the others are plants evolved (or divinely modified) to use mana in place of sunlight.
On a somewhat related note, I actually envision floor fungus as a lichen, not a pure fungus. Plump helmets and dimple cups might be lichens as well, actually.
Dimple cup: Of the images
Bumber showed, the yellow ones are closer to the shape I envisioned for dimple cups (a more convincing "cup", and closer to the heart glyph used to represent them in the ASCII tileset). As a side note,
mushroom dyes are quite real, and in the aggregate offer a fairly diverse color palette, even if dimple cups specifically are fictional.
Dimple dye is listed as "midnight blue" in descriptions (a dark purplish shade), but appears to me a darker cyan in the display (likely to avoid the problem of very dark blues blending into the black background). It is probably indigo or a derivative.
Plump helmet: My main visualization starting point is the "white" or "button" mushrooms you find in stores, but with the name "plump" I envisioned them as being much larger.
Quarry bush Sort of a cross between Romaine lettuce and dandelions: long leaves spreading out in all directions (probably in multiple tiersto earn the name "bush"). I use Romaine lettuce as the main base for the appearance of individual leaves.
I envision the seeds as being borne on a tall stalk for some reason; not quite sure why. The name "rock nuts" indicates that the seeds have a very hard shell (probably something like almond, acorns, or walnuts), and I envision this being opened and removed before the seeds are milled for oil.
Cave wheat: A member of the cereal grasses (wheat, rye, barley, etc., or possibly rice), adapted to the caverns but with largely superficial differences from its surface relatives.
Pig tails: Definitely a bast fiber (from the stem, like linen). I actually envision the "twisting stalks" as ideally trying to climb something (a stalagmite or one of the trees that grows in the caverns), and twisting around it. Without something to climb, the stem interlocks wth itself instead.
Sweet pods: My starting point when envisioning a "pod" os a pea pod. Sweet pods in DF would be more plump, and I envision them being squeezed to extract the syrup.
Cave moss: I envision this as being about as specific as "grass": a wide vvariety of species that are similar enough that the dwarves rarely need to distinguish them.
Also, apparently some moss species can grow up to a few inches tall, instead of just growing as a crust on the ground. This would increase the practicality of grazing by biting the plants off, instead of scraping off the moss with the tongue or something like that.
Surface plants:My impression of the surface plants was formed in v34.11, before most of the "real world" plants were added to the game in v40.01, and I have only partially re-evaluated since then. Given the relative lack of variety and the large fraction which grew almost anywhere, my envisioning started with the premise that, at least when it comes to surface plants, dwarves aren't very good botanists, so the species shown were probably large clusters lumped together based on superficial similarities.
Prickle berry: Blackberry or something similar: the berry itself isn't very prickly, but the vine it grows on is.
Longland grass: Could refer to any or all of the cereal grasses (wheat, barley, oats, rye, upland rice, possibly some of the millets, or even pseudocereals like buckwheat and amaranth grains), except maize, which is visually quite distinct.
Valley herb: I didn't really think about this one much, since I tend to embark in forests, where this plant doesn't grow.
Sliver barb: Similar, since I avoid evil areas (although if I had a better way to distinguish the ones that produce undead from the ones that don't, I might be more willing to try them).
Rope Reed: I assumed this was another bast fiber. From the name, I actually envisioned it as producing stems that could be used directly as cord (although not necessarily very strong cord) and narrow and flexible enough to be woven directly for some applications (baskets, for example), without the more complicated processing to produce proper thread.
Given the similarities to Pig Tail, Rope Reed may be the surface ancestor from which Pig Tail evolved or was developed. Possibly related to papyrus, but I haven't studied papyrus in much detail.
Whip Vine: As it happens, there actually is
a real life species of this name, although I'm not sure how similar it is to the DF version.
Hide Root: This obviously refers to, or at least includes,
madder, a group of plants once grown on a fairly large scale for the red dye that can be extracted from its roots. This dye was noted for a lack of colorfastness: DF doesn't actually render that property, but it could still serve as the reason behind its value being substantially lower than emerald and dimple dye.