I'll document this better in the manual later, but quick answer for now is that after encountering a noun, subsequent nouns are ignored unless a proposition is supplied. (English language behaviour, slightly different behaviour for Spanish mode)

If a word is a noun and an adjective, then it will read the first word as a noun, and if it discovers the next word is also a noun, then it will try to interpret the first word as an adjective.

1. EXAMINE ADDER'S TONGUE FERN
    ^
   MATCHED VERB = EXAMINE
2. EXAMINE ADDER'S TONGUE FERN
             ^
         Not in dictionary, ignored (internally removes ' char so actually looking for adders in dictionary)
3. EXAMINE ADDER'S TONGUE FERN
                     ^
                MATCHED NOUN1 = TONGUE
4. EXAMINE ADDER'S TONGUE FERN
                           ^
                      Matched Noun but as follows previous noun, discard. Also checks
                      to see if tongue is an adjective as well as a noun so it can
                      change tongue to an adjective if in dictionary.

The fix is to add tongue as an adjective to the fern:

start_at = room01
locations {
   room01 : location "You're in a storeroom";
}
objects {
   tongue      : object "some hound's tongue" start_at = "room01";
   tongue_fern : object "some adder's tongue fern" start_at = "room01";
}
on_command {
   : match "examine tongue" {
      : print "It's the tongue of a dog.";
      : done;
   }
   : match "examine fern" {
      : print "It's adder's fork.";
      : done;
   }
}

I will also probably change the noun discardation order so that if two nouns are encountered in a row and the first noun is not also an adjective, that the most recent noun overwrites the existing noun. I may also retain additional nouns and adjectives in a future version of the parser. 

I hope this answers your question, and I realise I have to document the parser better.

FYI - by switching DEBUG ON in the editor you can see how Adventuron is parsing the logical sentence. It's very spammy but sometimes useful.

Chris