Some background information on xanthorrhoeas
Traditionally, in the eastern states of Australia, plants in the genus Xanthorrhoea were often called by what are now believed to be racist common names that supposedly referred to an imagined resemblance to Australian Aboriginal people. It is not appropriate to refer to the genus by such names so now the formally adopted common name is Grasstree. This name refers to the superficial resemblance of the leaves to grasses. It is also not a particularly accurate name, I believe. Common names are often misleading and inaccurate as well as regularly misapplied so I believe the best common name for the genus is the term 'xanthorrhoeas'. That is unmistakeable and accurate.
In South Australia the most-used common name was Yacca. I do not know the origin of that name, but it may be the most original name of all.
There are many common myths about xanthorrhoeas. Two of the greatest myths are that (a) they are an ancient group of plants and, (b) that individual plants are many thousands of years old. In fact, xanthorrhoeas are a very advanced group of plants and they grow much more quickly than has been believed to date.
The myth about the supposedly ancient nature of the genus arises from a misunderstanding of their anatomy and structure. Cycads and tree ferns are both plant groups with a (very) superficially similar growth form to xanthorrhoeas and both plant groups are ancient. However, all these three groups have fundamentally different anatomical structure. I'm not going to elucidate the anatomy of cycads and tree ferns here but an easy search will show their structure.
I discuss Xanthorrhoea structure and anatomy briefly below and may do so in greater detail in future pages.
The myth about individual Xanthorrhoea plants being thousands of years old arises from a different logical error. A horticulturist in the 1950s germinated some Xanthorrhoea seeds and he observed that they produced from one and a half to two an a half leaves in the first year. He then counted up the old leaf bases on mature, trunked xanthorrhoeas and found that they numbered in the many thousands. He deduced, incorrectly, that meant that the plants, 50 cm to 60 cm high and with 6-8,000 leaf bases, were between 5,000 and 6,000 years old. What the researcher failed to understand is that the first few years of growth in xanthorrhoeas goes primarily into forming a substantial underground stem base and root system. Once the base and root system have formed the plants are producing from 150 to 250 leaves per year depending on the species and growth conditions. That means that the plants could in fact have been only 30 to 50 years old.
Some basic botany and anatomy
Xanthorrhoeas are monocotyledonous, which means they are one of the group of plants that has only one seedling leaf. The term is usually abbreviated as 'monocot'. This group of plants includes grasses and palms as well as more evolved members such as Agave, Aloe, Cordyline, Dracaena etc. along with Xanthorrhoea.
Plant stems are primarily made up of vascular tissue and the tissue that surrounds that vascular tissue. The vascular tissue called xylem carries the water and nutrients up from the roots to the leaves and other green parts of the plants and the tissue called phloem carries the sugar-rich sap produced by the process of photosynthesis that occurs in those green parts to feed the living parts of the plant. Most monocots do not develop true wood as they do not have secondary thickening. That is one of the greatest distinguishing features between monocots and dicotyledonous (dicots) plants. The dicots all have a more or less cylindrical layer of tissue called a cambium that generates new tissue (secondary thickening) and the 'tree rings' we all know about. Basically, the cambium develops water conducting tissue (xylem) on the inside of the plant and sugar/sap conducting tissue (phloem) on the outside. The xylem becomes the wood of the plant.
In contrast, most monocots have vascular bundles, which contain both xylem and phloem, scattered through a matrix of other stem tissue. Most monocot genera never develop any secondary tissue. However, xanthorrhoeas and some other advanced monocots do develop secondary thickening, though it is in a very different form to the secondary thickening seen in dicots.