Tasmanian Plants

Leaves of the Fragrant Candlebush become 'incontinent' with age

There are more similarities between plants and humans than we give credit for. Introducing the concept of incontinence in plants!

Leaves are the key parts of a plant which carry out the important act of water exchange with the environment. This being so, how effectively a plant can use water depends on how the easily water vapour gets from inside the leaf through the cuticle (the covering cell layer on the surface of a leaf) to the outside.

Greg Jordan and Tim Brodribb of the School of Plant Science, University of Tasmania have been studying the physiology of plant leaves for some time now and in 2007 published a paper in the scientific journal Functional Plant Biology on an interesting finding that the leaves of Agastachys odorata (Frangrant Candlebush or White Waratah) actually become incontinent with age.

Agastachys odorata is an endemic plant of the Protea family (Proteaceae) and occurs commonly in the high rainfall western Tasmania. The leaves of A. odorata exhibit distinctive annual growth increments, making different aged leaves easy to tell apart. The leaves are also very long-lived, with evidence of some leaves remaining on plants for up to 21 years. This makes A. odorata a fabulous choice for studying plant water relations with regards to leaf age.

Greg and Tim wanted to test the hypothesis that water vapour leaks across plant cuticles more readily as plant leaves age. In A. odorata they found that the older leaves were less effective in controlling water loss and hence used water less effectively than younger leaves. The increasing permeability of the leaf cuticle is implicated as the cause for this but it is also likely that the ability to control stomata opening also decreases with age, much akin to poor urinary sphincter muscle control in sufferers of incontinence. Greg and Tim concluded that the decreasing ability to use water efficiently could be due to natural leaf damage that occurs throughout the life of the leaf.

When the leaves of A. odorata becomes too old, it is simply shed. After all, the plant has no lack of leaves to serve it’s physiological functions. If only humans had the luxury of replaceable urinary sphincter muscles.

A rather recent trend in molecular science has been to use the technique to extort genes to reveal the history of how a plant has extended it’s geographical distribution throughout time.

I have written about how researcher James Worth used molecular techniques to pinpoint the locations (refugia) where Myrtle Beeches (Nothofagus cunninghamii) survived during the last glacial period. Just earlier this year, researchers Paul Nevill, Gerd Bossinger and Peter Ades published a paper in the Journal of Biogeography doing the same for the Mountain Ash (Eucalyptus regnans).

As in James Worth’s Myrtle Beech study, the researchers looked for variations at specific locations in the chloroplast DNA in Mountain Ash individuals distributed throughout the species natural geographical range. Different individuals may exhibit specific sequences which may differ from region to region and these are known as haplotypes.

A large amount of haplotypes found in a population an any given area would suggest that the area is a glacial refugium as we would expect a species to have persisted for longer periods of time in a refugium, thereby accumulating genetic changes. Conversely, places with low diversity of haplotypes could be construed to have been colonized after the glacial period ended, as there wouldn’t have been time enough for a high diversity of haplotypes to develop.

The results of the study showed that Mountain Ashes of the Northeast and Southeast of Tasmania has a high diversity of haplotypes, many of which were unique to the region. This suggests that the Northeast and Southeast of Tasmania harbored refugia that sheltered Mountain Ashes during the glacial period. By contrast, the central parts of Tasmania had a lower diversity of haplotypes. Another way of interpreting this was that there was fixing of haplotypes in that region, suggestive of a more recent colonization of the area following the end of the glacial period.

One consideration that remains to be addressed is the ease with which Eucalypts hybridize. E. regnans for example may hybridize with E. oliqua (Stringybark) and E. delegatensis (Gum-topped Stringybark). Hybridization may result in chloroplast sharing between species and a more comprehensive study will probably be needed to ensure that all these factors are taken into consideration.

For now it seems we are getting closer toward reading the the silent tale of survival that the ancestors of the Mountain Ashes in the Northeast and Southeast have etched in the genes of their descendants.

Hybridization as a means of making new species is not an uncommon concept and hence it must be applicable to other species. I present a case using a Tasmanian example – the Olearia daisybushes.

Olearia is a large and conspicuous genus of shrubs in the sunflower or daisy family (Asteraceae) with some 23 species in Tasmania of which 8 are endemic to Tasmania (not counting subspecies).

Left: Geebung Daisybush (Olearia persoonioides); Right: Prickly Daisybush (Olearia pinifolia); Centre: Possible hybrid

Two of the endemic species are of interest in this post: Olearia persoonioides (Geebung Daisybush) and Olearia pinifolia (Prickly Daisybush). Both are common and largish daisybushes that grow in subalpine woodlands.

Whilst botanizing at various spots around the Central Highlands I stumbled upon the two species of daisybushes growing in close proximity in the understorey of a eucalypt woodland. They were both in full flower. At the same time I also noticed numerous specimens that looked like intermediates between the two.

While this intermediate specimen deserves much more detailed study, I have prepared a set of photographs and made a table of the characters comparing the two daisybush species with the intermediate specimen.

Many additional aspects of the morphology of the intermediate specimen deserves study. For example, the morphology of the flowers and fruits (achenes) needs to be examined in greater detail. Other studies like chromosome counts might also be helpful in determining the hybrid status of the intermediate specimen.

A trip to the herbarium is in the works!

The Coral Heath (Epacris gunnii) is a fairly common shrub that may be found in wet heath to highland plateaus. This ornamentally attractive plant has heart-shaped leaves with a pointed tip and in it’s full flowering glory produces in a spike-like fashion, numerous flowers in the leaf axils.

In the Royal Tasmanian Botanical Gardens there are many cultivated plants of the Coral Heath, and in particularly, a double form that produces small Camelia-like flowers.

Even though I had the prior awareness the aberration of genetic mechanism of these double form plants, I was still pretty surprised when I stumbled on this strange phenomena of seeing a branch produce a ‘flower’, consisting of a whorl of petals, and have a new shoot growing out of the whorl of petals.

Just to be sure I even sliced the stem and ‘flower’ longitudinally to make sure and indeed, the new shoot just grew continuously out of the whorl of petals.

It is almost as thought the plant decided to make a flower but got sidetracked at the last minute and continued with vegetative shoot growth.

Makes one think, what exactly are flowers?

Many botanists must have pondered on this question.

Thankfully we have some theories.

Thus we learn in botany that flowers consists of four whorls of floral parts in the following order: sepals, petals, stamens (male parts) and carpels (female parts). All flowers are technically modifications of this scheme. And these whorls, some might be surprised to know, are actually modified leaves.

What might come as an even bigger surprise is that the theory of flowers being modified leaves was actually conceived over two centuries ago in the brilliant mind of Johann Wolfgang Goethe, the famous German poet and philosopher.

Goethe, in 1790, had no way of knowing the action of genes in the onset of flowering but his powers of observation would put many a scientist to shame. His insights were discussed his very aptly titled essay, Metamorphosis of Plants.

The concept of flowers being modified leaves might seem very abstract, particularly given the fact that flowers seem to be so different from leaves.

But therein lies the genius of plants. They transmutate. They morph. They make flowers from ‘leaves’. And here it seems our aberrant Coral Heath, leaves from flowers.

Ozothamnus antennaria (Sticky Everlastingbush)

In Tasmanian botany I was taught the concept of a cline, where a plant species seems to metamorphose into another species along an environmental gradient. In other words, what is considered a plant species at one end of a environmental continuum (eg, the base of a mountain) shows continuous morphological variation and seems to become another species as one goes up a mountain.

The most quoted and classical example of this would be that of the eucalypts, where you might see the Yellow Gum (Eucalyptus johnstonii) grading into the Alpine Yellow Gum (Eucalyptus subcrenulata), which grades into the Varnished Gum (Eucalyptus vernicosa) along some mountains in Tasmania.

I have often wondered how the concept of clinal variation might apply to other Tasmanian plants.

Ozothamnus rodwayi (Alpine Everlastingbush)

An example I had in mind was of the Tasmanian Everlastingbushes (Ozothamnus spp.).

In particular, the high altitude Sticky everlastingbush (Ozothamnus antennaria), Alpine everlastingbush (O. rodwayi) and Mountain everlastingbush (O. ledifolius) seem to exhibit morphological features that makes it easy to imagine that these species somehow evolved from one to the other or graded from one to the other along some sort of a environmental gradient.

It is easy to imagine the leaves of O. antennaria (which grows at slightly lower altitudes from it’s two relatives) becoming smaller and the flower heads getting more compact until it becomes something like O. ledifolius, the morphology of O. rodwayi being intermediate.

Ozothamnus ledifolius (Mountain Everlastingbush)

Not sure if this betrays any relationships: O. ledifolius smells vaguely of cinnamon spice and O. rodwayi seems to have a similar, albeit fainter smell. O. antennaria has the faintest smell last I took a sniff.

In any case, the idea of a cline in the everlastingbushes could be fallacious. But afterall, all good science starts with a conjecture, insane as it may seem. It would really be an interesting hypothesis to test using molecular methods, wouldn’t it.