Melaleuca aridus Simpson desert 2013 Following on from last week, this plant follows the theme of bird pollinated flowers from arid Western Australia. It is also a little controversial, and I have a conflict of interests. Until recently, the plant pictured above was generally considered to belong to the genus Calothamnus, closely related to another genus of shrubby plants, Melaleuca (below, right). For some time there was disagreement as to whether there was sufficient visible differences between these two groups to justify this separation. However using molecular sequence data for my honors project, I showed that as far as relationships go, these two groups of plants are mixed in together, and should be considered one and the same. Essentially Calothamnus are just slightly odd Melaleucas. Specifically, they have single flowers, rather than clusters, and these have squeezed their stamens together to form an elongated tube, the entrance to which is covered by a fringe of anthers (yellow capsules) holding the pollen. This is classic evolution toward specialized bird pollination, where the deep curved flower forces the bird to push it's bill well down into the nectaries, receiving a face-full of pollen for it's trouble. Compare this to the classical Melaleuca flower (below right) which has multiple flowers arranged in a cluster, each holding it's stamens much more openly, allowing access not only to birds but to insects and other animals as well. The whole cluster of flowers acting as a pollen brush. It is an interesting hypothesis that the evolution of more specialized bird pollinated flowers in the group-formerly-known-as-Calothamnus may be beneficial in moving pollen between the small, highly fragmented and widely dispersed populations of these plants. The nuts (seed capsules) formed from fertilized flowers are incredibly hard and thick-walled, providing excellent protection from the harsh environment and predation. Apart from being strikingly elegant, the flowers of this particular species (Melaleuca aridus) emerge as pale peachy pink (above left) and mature to the vibrant red over a day or so. As with most Myrtaceae they have no perceptible smell. The plants themselves grow to be lanky scrubby shrubs roughly 2m high with round pencil like leaves. Similar to last weeks plant (Leptosema) these Melaleucas have specialized leaves that are an adaptation to highly arid conditions, being cylindrical to minimize surface area and water loss, with a layer of furry hairs to further help insulate the leaf. Conserving every drop of water is important to a plant that grows on ridges and rocky rises surrounded by deserts of deep dry sand, a problem faced by many Australian plants, which have evolved many varied and fascinating ways of adapting to meet the challenge.
Photos: R.D. Edwards
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Leptosema chambersii
What happens if you are a small plant that is pollinated by birds or mammals rather than insects, but you have no branches for the animal to land on to access your flowers? Grow your flowers next to the ground so that they are easier to get to! Leptosema is a small genus of pea flowers native to semi-arid Australia that have done exactly this. Each small straggly plant produces bright red flowers around its base (red is more attractive to vertebrates than insects) where they are within easy reach of any animal on the ground. Flowers produce a rich nectar reward that is attractive to mammals and birds, and have been turned upside down so that as an animal pushes forward into the nectary it receives a brushing of pollen on the top of the head. This is subsequently transferred to the female parts (stigma - large curved rod structure seen here) of the next flower it visits. The tall narrow flowers inspired the name lepto (slender) sema (banner) from the Greek. Interestingly it appears that shifting from insect to bird pollination has evolutionary consequences, with reduced species diversity and potentially greater extinction risk: http://www.biomedcentral.com/1471-2148/14/43 Arid growing conditions have also led to the loss of true leaves (leaves loose water easily) in these species, and the development of flattened stems which hold chloroplasts and photosynthesize instead. Photo: R.D. Edwards Pterostylis nutants (nodding greenhood)
A native Australian terrestrial orchid, widespread throughout eastern Australia, favoring moist protected gullies in schlerophyll (meaning "dry leaves") open eucalypt forest. Like many orchids, this plant has an interesting way of being pollinated (pollination syndrome). An insect is attracted to the flower (presumably by scent or pheromones) and enters under the tongue ("labellum", curving out mid right) seeking a (nonexistent) reward. The tongue closes behind the insect forcing it to crawl up into the translucent windows, towards the light and then eventually through the larger chamber back down and out (anti clockwise from top here). On the way they must pass the pollinea (pollen bundles, black blob, extreme left) that stick to its body. The insect then pushes through the beak of petals/sepals at the bottom, leaving unrewarded, annoyed, loaded down with pollen, but presumably not having learned anything as they will fall for it all over again, this time fertilizing the next flower with their pollen passenger. Photo: R.D. Edwards |