can be fertile sources of humor 
(ROOT (VP (MD can) (VP (VB be) (NP (NP (JJ fertile) (NNS sources)) (PP (IN of) (NP (NN humor)))))))
(ROOT (VP (MD ) (VP (VB ) (NP (NP (JJ ) (NNS )) (PP (IN ) (NP (NN )))))))