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