As a child, petrified wood captured my imagination. However, as an adult, when someone taught me to look at the fossil wood at a microscopic level, I was in awe. At that moment, I like to think that I shared a joy similar to what the famous scientist, Robert Hooke, must have experienced when he examined fossil wood structure using his microscope, the first person ever to do so. The development of digital cameras and microscopes has catalysed my interest in using both technologies to zoom in on fossil wood specimens. In this respect, the purpose of this article is to stimulate this same interest among collectors.
The building blocks of life
In the last paragraph of The Origin of Species, Charles Darwin (1809-1882) eloquently reflects on the common ancestry of life on Earth.
“There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” (Darwin, 1859, p. 490)
Darwin recognised that there exists a continuity to life on Earth through his theory of natural selection. This same continuity is echoed in the work of the German physician, Rudolf Virchow (1821-1902). In his 1858 classic work, Die Cellularpathologie, Virchow enunciates an idea that would add a critical component to cell theory. The framework for cell theory had been suggested just 19 years earlier.
Mattias Jackob Schleiden (1804-1881), a German botanist, established that all plants are composed of cells. Theodor Schwann (1810-1882), a German physiologist, combined Schleiden’s work on plants with his own observations that all animals are composed of cells, to set out a cell theory published in his 1839 book, Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants (Schwann, 1839/1847, pp 186-215). Schwann and Schleiden’s cell theory proposed that the cell is the basic unit of structure and function for all life. They suggested cells arise and grow through a process not unlike cystal growth. The crystallisation process was hypothesised to occur either within or on the outside surface of cells. It is this nature of how cells arise that Virchow revised and corrected. He realised that cell division could account for cell reproduction.
As Virchow wrote, “Wo eine Zelle entsteht, da muss ein Zelle vorausgegangen sein (Omnis cellula e cellula)…” (Virchow, 1859, p. 25; “Where a cell arises, it must have been preceded by a cell”). With his emphases in Latin that all cells arise from cells, Virchow added an important tenet to Schleiden and Schwann’s earlier work completing what many high school biology students learn as the cell theory to this day: all living things are made of cells, cells are the units of structure and function for life, and cells come only from pre-existing cells (Miller and Levine, 2010, p.191).
The cell doctrine or cell theory is a cornerstone of modern biology. In the light of Darwinian evolution, cell theory indicates common ancestry through cell division. Christian de Duve, in his book Life Evolving: Molecules, Mind, and Meaning, uses a simple thought experiment, modified here, to contemplate how life is connected through deep time at the cellular level (deDuve, 2002, pp. 9-10). In adult humans, each of the more than 1013 cells can be traced back to the original unicellular zygote. The zygote itself was the product of a sperm fertilising an egg cell. The sperm and egg cell can be traced back to the zygotes from which they arose. This simple thought experiment takes us back from one generation to the next. There exists an unbroken continuity to life, such that we can trace all of our cells back to the very first cells that existed on Earth. Thus, we can infer from the cell doctrine that all organisms can trace their cells back to the very first cell from which all life arose some 3.5bya.
Amazingly, the first use of the term ‘cell’ and an account of the microscopic structure of fossil wood occurred over 200 years before Virchow’s work. Robert Hooke (1635-1703), the English natural philosopher referred to above, christened the term cell in his book Micrographia to desribe “little boxes” making up a pattern not unlike an empty “honey-comb” within a thin section of cork examined through his microscope (Hooke, 1665, p. 113). The cells that Hooke observed were empty and dead; he did not realise the importance that these structures have to life. In fact, it was Anton von Leeuwenhoek (1632-1723), a Dutch scientist, who was the first person to see and illustrate single living cells using his microscope.
Interestingly, just four pages before naming the basic unit for all life, Hooke became the first person to hypothesise how wood might become mineralised. Hooke was asked by the Royal Society to examine petrified wood using his microscope (Hooke, p. 107). He came to the conclusion that petrified wood exhibits the same structure as living wood and suggested a process by which living wood might turn into the nature of stone:
“That this petify’d Wood having lain in some place where it was well soak’d with petrifying water (that is such a water as is well impregnated with stony and earthy particles) did by degrees separate, either by straining and filtration, or perhaps, by precipitation, cohesion or coagulation, abundance of stony particles from the permeating water, which stony particles, being by means of the fluid vehicle convey’d, not onely into the Microscopical pores, and so perfectly stoping them up, but also into the pores or interstitia, which may, perhaps, be even in the texture or schematisme of that part of the Wood, which, through the Microscope, appears most solid, do thereby so augment the weight of the wood, as to make it above three times heavier than water, and perhaps, six times as heavie as it was when wood.” (Hooke,1665, p. 109).
At the time Hooke wrote this description, many people were unsure about the nature of fossils. Hooke came to the conclusion that once-living trees turn to stone when mineral ladened water permeates the buried wood. Modern studies of artificial and natural silicification of wood support the idea that multiple pathways can lead to the formation of mineralised wood. However, the idea that mineral laden water is part of the process is incorporated into every model (Mustoe 2015; Dietrich, Viney, and Lampke 2015).
Since Hooke’s observation of fossil wood, there have been important advances in the techinques used to observe fossil wood. Henry Witham (1779-1844) was an Englishman who pioneered the use of thin sections to study the internal microstructure of fossil plants. In his groundbreaking work, Observations of Fossil Vegetables, Witham describes how to make transverse and longitudinal thin sections of fossil wood to observe cell structure (Witham, pp. 187-189). To this day, making thin sections in transverse, radial and tangential orientations remains a standard practice for identifying fossil wood types. Scanning electron microscopy (SEM) was first used extensively by Peter Buurman to examine cell structure in mineralised wood (Buurman, 1972). Fossil wood specimens prepared for thin section slides or SEM studies must be altered by cutting, cleaving and fracturing. These techniques require skills and equipment that may not be available to the average person. However, cell structure that is visible along natural surfaces of fossil specimens or those prepared by lapidarists can be observed and enjoyed using equipment that requires very little training and which is accessible to the average person. The good news for a collector is that no alteration to their treasured fossil is required.
Mineralised dinosaur bone and plant material that exhibits cell structure provide opportunities to photograph and contemplate lifes’ fundamental building blocks preserved in stone. Robert Hooke and Anton von Leeuwenhoek established and popularised the study of life at a microscopic level with their discoveries and carefully made drawings. Just as Hooke and his colleagues discovered a new world with the advent of microscopy, contemporary students of fossil wood and dinosaur bone can explore this same world using affordable digital cameras and microscopes. In fact, we can use macrophotography combined with microphotography in a non-distructive way to zoom in on a specific area revealing what the unadided eye cannot observe.
Macro to micro photography
Our first specimen is the silicified tree fern, Titea singularis, from the Permian age Pedra de Fogo Formation in Bieland, in the Maranhao Province of Brazil (Figs. 2, 3, 4 and 5).
The cross-section of this fern tree exhibits a central vascular cylinder surrounded by a thick root mantle (Fig. 2). As we zoom in to see the boundary between the vascular cylinder and the inner root mantle, individual adventitious roots with star-shaped centres become visible (Fig. 3).
We switch to using the Dino-Lite AD7013 MT 5.0 MP for the last two figures (Figs. 3 and 4). Fig. 3 zooms in at 60x on several adventitious roots near the central vascular bundle. Roots are organs made of specialised tissues that are, in turn, made of specialised cells.
In Fig. 4, we zoom in on the cross section of a single adventitous root magnified 105 times. The root measures 2mm by 2.5mm in cross-section. The star-shaped structure represents xylem, a tissue specialised for water transport in plants. The xylem of ferns is composed of tube-shaped water conducting cells called tracheids and vessel members. The largest xylem cells making up the star-shaped centre in this image measure just under 100μm in diameter.
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Mike Viney (UK)