[tag=NNS] [tag="-LRB-"] [lemma=van] [□]
[tag=NNS] [tag="-LRB-"] [lemma=van] [□=□]
[tag=NNS] [tag="-LRB-"] [lemma=van] [□ | □]
Therefore , aquatic " propagule banks " can be diverse and complex , containing up to 29 species ( van der Walk and Davis 1976 ) .