Plants often change colour dramatically in autumn or winter from green to a distinctive deep red colour. The colouration is caused by the production of anthocyanins, a process that is controlled by environmental conditions and/or by stress.
In common with other ferns, Azolla has 2 alternating generations:
Azolla is one of the few ferns that are heterosporous - they produce 2 types of spores, large female spores and tiny male spores. These form inside sporangia contained in sporocarps. The sporocarps are able to withstand sub-zero temperatures as well as desiccation, so can help populations to overwinter and survive adverse conditions. However, Azolla does not regularly produce spores in the UK, a process probably influenced by environmental factors.
Fluorescence image of cross-section of the megaspore apparatus of Azolla mexicana.The megaspore is on the left with part of the spore wall; to the right are two of the triangular floats; on the extreme right is the brown indusium cap below which (seen as red dots) are the cyanobacteria. © H Schneider
The massula glochidia get caught in the hairs of a floating megaspore and the combined structure sinks. Fertilisation occurs and the resulting zygote (fertilised egg) develops into a new sporophyte the following spring.
Azolla species are unique in that the cyanobacterium symbiont is transferred from generation to generation via the megasporocarp, rather than the new sporophyte requiring reinfection by the cyanobacterium.
The relationship is mutually beneficial as the cyanobacterium has a niche to live in and the Azolla receives the nitrogen it requires.
Vegetative propagation in Azolla is extremely efficient. Plants break up very readily at branching points to establish numerous new individuals. If conditions are favourable, Azolla filiculoides can double the area it covers in just 5 to 10 days.
Plants usually die back or sink in winter, though they can overwinter in milder climates. In spring when temperatures rise the plants begin to grow and rise to the surface.
Rich in nutrients.
Barbed hairs on massulae.
Having 2 kinds of spores.
Tissue that protects the sporangia.
Mass of microspores and associated tissue (term is also applied to female ‘floats’ in the megaspore apparatus).
Descended from a single common ancestor.
Minute projection on the surface of a leaf.
Structure that contains the sporangia.
An organism in a close and long-term association with another species.