Sugar Gum under the Microscope

During 2008 the NUFG became increasingly interested in understanding the biological aspects of plantation trees within the Kamarooka Project site. We became particular interested in understanding the hydraulic functions and mechanisms involved in interactions between the trees and shallow saline groundwater.  

The watertable beneath the plantation had begun to recede when the trees were only three years old. When they were four years old NUFG installed sap flow monitoring that allowed us to observe daily water use. This was an exciting time in which there was much reading of the literature. In addition, discussions with several experts enabled us to develop a working knowledge of how the trees function during photosynthesis.  We began to appreciate how the leaves manage to draw water up from their roots, and how they do this in spite of very large osmotic pressures imposed by saline soils and saline groundwater. 

Felling E. cladocalyx for electron microscope samples

This working knowledge of how the NUFG trees function led to an appreciation of the internal hydraulic structure that allows water to pass from the roots up through the stem to the leaves. We came to understand that evapotranspiration from the leaves causes water to pass up the trees through xylem tubes present within the sap wood. 

The structure of the xylem was something we could read about in the literature and we could discuss in abstract terms at the NUFG meetings, but word pictures were no substitute for microscopic images.  Accordingly, in the latter half of 2008 we approached Dr. Sabine Wilkens at Latrobe University to see if she might help with attaining scanning electron microscope images for the Kamarooka Sugar Gums.

The contact with Sabine occurred after a presentation by Phil Dyson on the NUFG Kamarooka Project to the Bendigo Field Naturalists. Sabine was given the task of writing up the notes for the Field Nats magazine and subsequent discussions with Phil soon led to comparisons with knowledge gained from her PhD thesis on the functional biology of mangrove trees.  

NUFG supplied Sabine with sample of fresh Sugar Gum stem and she enlisted the support of 
Dr, Rob Glaisher at Latrobe University, Bendigo, an expert on the scanning electron microscope. Together they prepared the samples and captured the images shown below. 

Scanning electron microscope images of the xylem of four year old E. cladocalyx growing on saline land within the NUFG plantation at Kamarooka

The larger conduits seen in the images above are 'xylem tubes that carry water from the tree roots up to the leaves. Note the scale on each image. One micro-metre is a millionth of a metre and, accordingly, 100 um is one tenth of a millimetre. The xylem tubes are comparable in diameter to the average human hair. 

Note that in the higher magnification image on the right a regular series of perforations appear on the innermost parts of the xylem tube. Interestingly, these are not pores that allow for water passage across the structure.  Instead, they are a relict of the final stage of construction of the tube. The outer structure is constructed first as a continuous conduit, and later an inner lining is laid down as a helix (coil). The tree then fills in most, but not all, of the gaps in the helix. We see the remaining gaps as these small pores.

In Eucalyptus  trees, unlike conifers, the xylem tubes are continuous.

The mass of much smaller tube-like structures surrounding the larger xylem conduits are fibres that add strength to the wood structure.        

Side on view

Electron microscope images showing a longitudinal (side) view of the wood

The images above show a longitudinal view in which the xylem tubes appear as the larger tubes with a segmented appearance. They occur amidst a myriad of small circular cells stacked in single or double rows. These are known as 'rays'. They are living tissue that most probably transports material across the stem (outer to the inner).   

A closer look at the images, particularly the image on the right, reveals long thin grey cells that run along the length of the trunk. These are fibres. They do not transport water nor are they connected to one another.  They simply add supporting strength to the structure.