Eucalypt flowers and the land of wildfires

One of the best things about my job is going to the field and last week this was made possible, my mission was to visit a eucalypt orchard and collect capsules of Eucalyptus globulus. Perfect! Since I had to collect capsules from the canopy, and eucalypt trees are quite tall, the orchard manager and me had to go in a crane to reach the top of the trees, I couldn’t believe it! It was fantastic and it truly made my day. Even though I have been in even taller canopies before, of different species, in different places and situations, it was for me as marvelous as the first time. It is always fascinating to reach a tree perspective from its canopy – sorry for not having pictures of the moment, but I had to share this with you anyway.

As I was there for capsule collection, I obviously took the chance of collecting some flowers too. I have been looking for eucalypt flowers for quite a while, but the canopies are tall, so I never managed to reach them (I don’t always find a crane next to the trees for a little canopy ride). Eucalypt flowers might not be extremely spectacular in terms of pollination, but their morphology is very interesting – and you have to agree it’s hard to stop looking at these beauties.

Eucalypt flowers diversity: 1 – Eucalyptus rhodantha; 2 – E. kingsmillii; 3 – E. synandra. 
(Photos: https://www.flickr.com/photos/tgerus/)

Since eucalypts belong to Myrtaceae, you do not expect a perianth here, stamens perform the attractive parts of the flower, and the colour attributed to the flower is actually the color of the stamens’ filaments. But even if you are not expecting to find a perianth, it doesn’t mean they don’t have one, or had… Eucalyptus’ perianth is replaced by a protective woody structure – the operculum, which falls at anthesis.

The operculum is maybe the most emblematic structure of the flowers of this genus, but it is not a new structure, it is a modified one instead. The origin of the operculum is actually on the fusion of perianth members (calyx and corolla), but in most groups (e.g. Symphyomyrtus subgenus) you will find two opercula in one flower bud: an outer operculum (of sepal origin) and an inner operculum (of petal origin). In these species, the sepal-derived operculum falls during bud development and the petal-derived one falls at anthesis. So, perianth exists, but is only there on early stages, before anthesis, leaving only a scar on the flower as a reminder of their existence.  

E. globulus flowers; The presence of a scar in the flower bud is a clear evidence that the outer operculum has fallen. Species with a single operculum, or fused opercula, lack this scar. (Photo: Bill Higham)

Another character of Myrtaceae, which has been discussed in another post is the hypanthium. The hypanthium is present in a number of groups, its morphology is labile, as well as the tissues involved, so do not expect to find identical hypanthia in different groups. Eucalypt hypanthia embed the inferior ovaries, and only the style is visible, between the stamina ring and base of the style.

E. globulus buds and flowers at early and later stages.
(Photos: Forest and Kim Starr)

In a later stage, this receptacular structure is involved in the formation of the capsule. Since hypanthium and ovary are structures intimately linked, maturation of the ovary takes place inside the hypanthium and a woody dehiscence capsule is formed. This is why eucalypt capsules are known to be false fruits, because the hypanthium is involved in the process and in the fruit structure. Finally, dehiscence takes place after capsules dry out (usually as a response to dead tissues caused by fire), and capsule opens through valves formed by the splitting of the ovary roof, corresponding to the locules.

When capsules dry out, the ovary roof splits in several valves and seeds are shed through these valves. The number of valves correspond to the number of ovary locules. Scars and rings, corresponding to floral organs, are also still visible in mature capsules. (Photo: John Tann)

Considering the dehiscence behavior and the ecology of eucalypts, I wonder if the formation of these false fruits are actually adaptations to fire. It has been already showed that eucalypt seeds are not fire-resistant, so seed viability relies on the insulating properties of the woody capsules. Perhaps two layers of tissue, the hypanthium combined with the ovary wall, provide higher insulating capacity?