aerenchyma
noun
aer·en·chy·ma ˌer-ˈeŋ-kə-mə 
: modified parenchymatous tissue having large intracellular air spaces that is found especially in aquatic plants where it facilitates gaseous exchange and maintains buoyancy
Word History
Etymology
aer- + -enchyma, after German Aërenchym
First Known Use
circa 1893, in the meaning defined above