In fact, it does become 'boze'. :) De vrouw is boos. - Zij is een boze vrouw. 'Wortel' becomes 'wortelen' and not 'wortellen' because the last syllable of 'wortel' is not stressed. In that case, the plural usually only gets a single consonant. You can find a more thorough explanation (in Dutch) at http://taaladvies.net/taal/advies/vraag/222/

This is coming from a dutch learner (so don't take it as 100% - but nobody else answered) Regarding the boos/bose thing, a random thought - the s doesn't become a z because the word is an adjective. The S=>Z rule only applies to nouns i think. (huis-huizen) And there aren't many adjectives that end in S - The only that come to my mind are paars/paarse (used as an adjective) and fris/frisse ... As for wortel/wortelen .. i don't think there is any rule about doubling the consonant : gevoel/gevoelen ; doel/doelen ; twijfel / twijfelen ... however : aanval / aanvallen

And in fact, a related question - why does 'wortel' become 'wortelen' when adding -en and not 'wortellen'?