The pore of the leaf cavity of Azolla: Interspecific morphological differences and continuity between the cavity envelopes

Interspecific morphological differences between mature leaf cavity pores of the different Azolla species were investigated by light and low temperatur e scanning electron microscopy. Results indicated that the pore morphology constitutes a novel taxonomic criterion allowing us to separate the Azolla and Rhizosperma sections. Scanning documents also gave further insight into the pore function: it ensures gaseous exchanges between the environment an d the cavity, defends the cavity from external ingress and contributes to t he maintenance of the endosymbionts within this cavity. Transmission electr on microscopy of the cavity in the pore region indicated that the previousl y described one-layered inner envelope and three-layered outer envelope, bo th limiting the mucilage containing the endosymbionts, are not separate ent ities: they join together at the pore periphery and the inner envelope is a ctually constitutive of one of the layers of the outer envelope. Our observ ations allowed us to explain for the first time the possible origin and nat ure of each of the three constitutive layers of the latter envelope.