hence [came/we see/etc] the capital importance of arabesques, of geometrical and botanical decorative motifs.
In parsing, it remains a clause with an understood verb, I think, though on paper we are left with nothing but an adverb and a noun phrase. Still, clauses without overt verbs are rare.
Because there are so many different ways to analyze such structures, I am always hesitant to answer such questions. One never knows whether the student is studying from a book which takes a "traditional" approach to grammar and its terminology or from a more "transformational grammar" approach. I have come to find the transformational approach the more insightful approach over the years,