The structures of photosynthetic apparatuses such as leaves, chloroplasts and
symbiotic cyanobacterum (blue-green algae) in Azolla-Anabaena azollae associations (Azolla
imbricata (Roxb) Nakai) which occur in paddy fields of China were examined using light,
scanning and transmission electrn microscopy. Some comparisons were made with A.
filiculoides, A. japonica, A. caroliniana, A. pinnata and A. mexicana.
Cross sections of A. imbricata were observed by light microscopy and the symbiotic as-
sociation between the eukaryotic water fern and its prokaryotic blue-green algal symbiont, an
Anabaena, was studied. The symbiotic cyanobacterum cells occur not only in a mature leaf
cavity, but also in early stages of leaf development, around leaf primordia, and even in ma-
crospores.
Under scanning electron microscopy (SEM) it is possible to see stomata and nipples on
the surface of dorsal lobes of the fern. The species in the subgenus Euazolla (i.e.A. filicu-
loides, A. japonica, A. caroliniana and A. mexicana) have rounded nipples, but those in the
subgenus Rizosperma (i.e.A. imbricata and A. pinnta) prolate ones. This morphological
character is first reported to be related to the taxonomic system.
The result of the observation with transmission electron microscopy (TEM) shows
that A. filiculoides contains more thylakoides in chloroplasts than A. imbricata does, and the
grana lamellae have more stacks in the former than in the latter. The differences are in ag-
reement with the differentiation of the two species in photosynthetic capacity. This may be
one of the differences between the two subgenera.
The ultrastructures of the symbiotic cyanobacterum are similar to those of free-living
Anabaena. The vegetative cells show a typical bilayered cell wall and the heterocysts have a
thikened wall. The thylakoid membranes in both heterocysts and vegetative cells are often
seen forming whirls. During the division of vegetative cells, their contents aggregate and then redistribute.
