P. Veys et al., The pore of the leaf cavity of Azolla: Interspecific morphological differences and continuity between the cavity envelopes, SYMBIOSIS, 29(1), 2000, pp. 33-47
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.