In Velella the pneumatophore becomes of complex structure and sends air-tubes, lined by a chitin and resembling tracheae, down into the compact coenosarc, thus evidently serving a respiratory as well as a hydrostatic function.
The thin-walled spiral or annular tracheae of the protoxylem allow of longitudinal stretching brought about by the active growth in length of the neighboring living parenchymatous cells of a growing organ.
Hence such tracheae are only laid down in organs whose growth in length has ceased.
The fibres are frequently found in tangential bands between similar bands of tracheae or sieve-tubes.
Yarrell, " Observations on the tracheae of Birds," Trans.