Experimental science & plant physiology
Botany from the time of Theophrastus could be divided straightforwardly into pure and applied domains. Early natural history had created three major botanical streams morphology (classification), anatomy and physiology – that is, external form, internal structure, and functional operation, while the three most obvious streams in applied botany were horticulture, forestry and agriculture – although from now on disciplines began to emerge that did not fall into such neat categories as technology has opened up new techniques and widened the scope of study: weed science, ethnobotany, plant pathology, pharmacognosy, and economic botany and which sit uneasily, if at all, in modern plant science. Specialists now began to confine their interest to the botany of particular plant groups phycology (algae), pteridology (ferns), mycology (fungi, before these were placed in aseparate kingdom), bryology (mosses and liverworts) and palaeobotany (fossil plants).
Classifying plants was for the most part the routine process of descriptive science but the first half of the 18th century marked a move into experimental science – an examination of the way plants functioned and interacted with their environment over many scales from the large-scale global distribution and biological significance of vegetation and plant communities (biogeography and ecology) to the small scale processes operating within the plant as revealed by new subjects like cell theory, experimental physiology, molecular biology and plant biochemistry.
In plant physiology research interest was focused on the movement of sap and the absorption of substances through the roots. Jan Helmont (1577–1644) by experimental observation and calculation, noted that the increase in weight of a growing plant cannot be derived purely from the soil, and concluded it must relate to water uptake. Englishman Stephen Hales (1677–1761) established by quantitative experiment that there is uptake of water by plants and a loss of water by transpiration and that this is influenced by environmental conditions: he distinguished ‘root pressure’, ‘leaf suction’ and ‘imbibition’ and also noted that the major direction of sap flow in woody tissue is upward. His results were published in Vegetable Staticks (1727) He also noted that ‘air makes a very considerable part of the substance of vegetables’. English chemist Joseph Priestley (1733–1804) is noted for his discovery of oxygen (as now called) and its production by plants. Later Jan Ingenhousz (1730–1799) observed that only in sunlight do the green parts of plants absorb air and release oxygen, this being more rapid in bright sunlight while, at night, the air (CO2) is released from all parts. His results were published in Experiments upon vegetables (1779) and with this the foundations for 20th century studies of carbon fixation were laid. From his observations he sketched the cycle of carbon in nature even though the composition of carbon dioxide was yet to be resolved. Studies in plant nutrition had also progressed. In 1804 Nicolas-Théodore de Saussure’s (1767–1845) Recherches Chimiques sur la Végétation was an exemplary study of scientific exactitude that demonstrated the similarity of respiration in both plants and animals, that the fixation of carbon dioxide includes water, and that just minute amounts of salts and nutrients (which he analysed in chemical detail from plant ash) have a powerful influence on plant growth.
The nineteenth century saw major advances in plant physiology, mostly through the research of German botanists determined to elucidate water and nutrient transport through the plant. Much of the work of this period especially was carried out in the laboratories of Julius Sachs (1832-1897) and synthesised in his book Vorlesungen über Pflanzenphysiologie (1882).
Carbon fixation (photosynthesis)
At the start of the 19th century the idea that plants could synthesise almost all their tissues from atmospheric gases had not yet emerged. The energy component of photosynthesis, the capture and storage of the Sun’s radiant energy in carbon bonds (a process on which all life depends) was first elucidated in 1847 by Mayer, but the details of how this was done would take many more years. Chlorophyll was named in 1818 and its chemistry gradually determined, to be finally resolved in the early 20th century. The mechanism of photosynthesis remained a mystery until the mid-19th century when Sachs, in 1862, noted that starch was formed in green cells only in the presence of light and in 1882 he confirmed carbohydrates as the starting point for all other organic compounds in plants. The connection between the pigment chlorophyll and starch production was finally made in 1864 but tracing the precise biochemical pathway of starch formation did not begin until about 1915.
Significant discoveries relating to nitrogen assimilation and metabolism, including ammonification, nitrification and nitrogen fixation (the uptake of atmospheric nitrogen by symbiotic soil microorganisms) had to wait for advances in chemistry and bacteriology in the late 19th century and this was followed in the early 20th century by the elucidation of protein and amino-acid synthesis and their role in plant metabolism. With this knowledge it was then possible to outline the global nitrogen cycle.
Other discoveries and studies included osmosis and geotropism.
Advances in economic botany have benefitted from basic botanical research into plant physiology, genetics and so on, but empiricism is not confined to universities. Practical applied knowledge could be improved and made more efficient by constant critical observation noting what worked and what did not. Plants have always been the source of energy for our bodies so food has never been of secondary importance.
Perhaps unknowingly today’s familiar staple foods were all domesticated in prehistory. Seed would be collected from the high-yielding plants leading to the selection of higher-yielding varieties. Plants like peas and beans (legumes) were cultivated on all continents but cereals made up most of the basic diet on all continents except possibly Australia. There was rice in East Asia, maize in southern and central America, wheat and barley in the Middle east supplemented by local foods. In Greco-Roman times cereals were supplemented in the Mediterranean by grapes, apples, figs, and olives, Roman manuscripts already alluding to particular cultivated varieties of these plants. Botanical historian William Stearn has observed that ‘cultivated plants are mankind’s most vital and precious heritage from remote antiquity‘.
Though many Greek manuscripts were written on the subject of farming it was the practical Romans who left the founding texts from which later industrial agriculture would emerge.
Plant reproduction 1700-1800
Tracing the finer detail of plant sexuality requires not only good analytic kills but careful microscopic observation and only in 1694 was it conclusively shown that ovule development needed fertilization by pollen from the stamens finally confirming observations made years before by Babylonians observing date palms in Assyria in at least 885-860 BCE, Empedocles (490-430 BCE) and Theophrastus (371-287 BCE). From these early observations work across plant groups revealed the ‘alternation of generations’ and opened up the field of comparative morphology leading in the early19th century to an understanding of nectar and the role of insects and wind in pollination.
Nineteenth century foundations of modern botany
In about the mid-19th century scientific communication changed. Until this time ideas were largely exchanged by reading the works of authoritative individuals who dominated in their field: these were often wealthy and influential “gentlemen scientists”. Now research was reported by the publication of “papers” that emanated from research “schools” that promoted the questioning of conventional wisdom. This process had started in the late 18th century when specialist journals began to appear. Even so, botany was greatly stimulated by the appearance of the first “modern” text book, Matthias Schleiden’s (1804–1881) Grundzüge der Wissenschaftlichen Botanik, published in English in 1849 as Principles of Scientific Botany. By 1850 an invigorated organic chemistry had revealed the structure of many plant constituents. Although the great era of plant classification had now passed the work of description continued. Augustin de Candolle (1778–1841) succeeded Antoine-Laurent de Jussieu in managing the botanical project Prodromus Systematis Naturalis Regni Vegetabilis (1824–1841) which involved 35 authors: it contained all the dicotyledons known in his day, some 58000 species in 161 families, and he doubled the number of recognized plant families, the work being completed by his son Alphonse (1806–1893) in the years from 1841 to 1873.
Plant geography and ecology 1800-present
The opening of the 19th century was marked by an increase in interest in the connection between climate and plant distribution. Carl Willdenow (1765–1812) examined the connection between seed dispersal and distribution, the nature of plant associations and the impact of geological history. He noticed the similarities between the floras of N America and N Asia, the Cape and Australia, and he explored the ideas of ‘centre of diversity’ and ‘centre of origin’. German Alexander von Humboldt (1769–1859) and Frenchman Aime Bonpland (1773–1858) published a massive and highly influential 30 volume work on their travels; Robert Brown (1773–1852) noted the similarities between the floras of S Africa, Australia and India, while Joakim Schouw (1789–1852) explored more deeply than anyone else the influence on plant distribution of temperature, soil factors, especially soil water, and light, work that was continued by Alphonse de Candolle (1806–1893). Joseph Hooker (1817–1911) pushed the boundaries of floristic studies with his work on Antarctica, India and the Middle East with special attention to endemism. August Grisebach (1814–1879) in Die Vegetation der Erde (1872) examined physiognomy in relation to climate and in America geographic studies were pioneered by Asa Gray (1810–1888).
Physiological plant geography, perhaps more familiarly termed ecology, emerged from floristic biogeography in the late 19th century as environmental influences on plants received greater recognition. Early work in this area was synthesised by Danish professor Eugenius Warming (1841–1924) in his book Plantesamfund (Ecology of Plants, generally taken to mark the beginning of modern ecology) including new ideas on plant communities, their adaptations and environmental influences. This was followed by another grand synthesis, the Pflanzengeographie auf Physiologischer Grundlage of Andreas Schimper (1856–1901) in 1898 (published in English in 1903 as Plant-geography upon a physiological basis translated by W. R. Fischer, Oxford: Clarendon press, 839 pp.)
Developmental morphology and evolution
Until the 1860s it was believed that species had remained unchanged through time: each biological form was the result of an independent act of creation and therefore absolutely distinct and immutable. But the hard reality of geological formations and strange fossils needed scientific explanation. Charles Darwin’s On the Origin of Species (1859) replaced the assumption of constancy with the theory of descent with modification. Phylogeny became a new principle as ‘natural’ classifications became classifications reflecting, not just similarities, but evolutionary relationships. Wilhelm Hofmeister established that there was a similar pattern of organization in all plants expressed through the alternation of generations and extensive homology of structures.
Polymath German intellect Johann Goethe (1749–1832) had interests and influence that extended into botany. In Die Metamorphose der Pflanzen (1790) he provided a theory of plant morphology (he coined the word “morphology”) and he included within his concept of ‘metamorphosis’ modification during evolution, thus linking comparative morphology with phylogeny. Though the botanical basis of his work has been challenged there is no doubt that he prompted discussion and research on the origin and function of floral parts. His theory probably stimulated the opposing views of German botanists Alexander Braun (1805–1877) and Matthias Schleiden who applied the experimental method to the principles of growth and form that were later extended by Augustin de Candolle (1778–1841).
20th century science grew out of the solid foundations laid by the breadth of vision and detailed experimental observations of the 19th century. A vastly increased research force was now rapidly extending the horizons of botanical knowledge at all levels of plant organization from molecules to global plant ecology. There was now an awareness of the unity of biological structure and function at the cellular and biochemical levels of organisation. Botanical advance was closely associated with advances in physics and chemistry with the greatest advances in the 20th century mainly relating to the penetration of molecular organization. However, at the level of plant communities it would take until mid century to consolidate work on ecology and population genetics. By 1910 experiments using labelled isotopes were being used to elucidate plant biochemical pathways, to open the line of research leading to gene technology. On a more practical level research funding was now becoming available from agriculture and industry.
In 1903 Chlorophylls a and b were separated by thin layer chromatography then, through the 1920s and 1930s, biochemists, notably Hans Krebs (1900–1981) and Carl (1896–1984) and Gerty Cori (1896–1957) began tracing out the central metabolic pathways of life. Between the 1930s and 1950s it was determined that ATP, located in mitochondria, was the source of cellular chemical energy and the constituent reactions of photosynthesis were progressively revealed. Then, in 1944 DNA was extracted for the first time. Along with these revelations there was the discovery of plant hormones or “growth substances”, notably auxins, (1934) gibberellins (1934) and cytokinins (1964) and the effects of photoperiodism, the control of plant processes, especially flowering, by the relative lengths of day and night.
Following the establishment of Mendel’s laws, the gene-chromosome theory of heredity was confirmed by the work of August Weismann who identified chromosomes as the hereditary material. Also, in observing the halving of the chromosome number in germ cells he anticipated work to follow on the details of meiosis, the complex process of redistribution of hereditary material that occurs in the germ cells. In the 1920s and 1930s population genetics combined the theory of evolution with Mendelian genetics to produce the modern synthesis. By the mid-1960s the molecular basis of metabolism and reproduction was firmly established through the new discipline of molecular biology. Genetic engineering, the insertion of genes into a host cell for cloning, began in the 1970s with the invention of recombinant DNA techniques and its commercial applications applied to agricultural crops followed in the 1990s. There was now the potential to identify organisms by molecular ‘fingerprinting’ and to estimate the times in the past when critical evolutionary changes had occurred through the use of ‘molecular clocks’.
Computers, electron microscopes and evolution
Increased experimental precision combined with vastly improved scientific instrumentation was opening up exciting new fields. In 1936 Alexander Oparin (1894–1980) demonstrated a possible mechanism for the synthesis of organic matter from inorganic molecules. In the 1960s it was determined that the Earth’s earliest life-forms treated as plants, the cyanobacteria known as stromatolites, dated back some 3.5 billion years.
Mid-century transmission and scanning electron microscopy presented another level of resolution to the structure of matter, taking anatomy into the new world of “ultrastructure”.
Biogeography, ecology, domesticated plants 1900-1950 ->
Colonial expansion had, for the colonists, made the world seem smaller and less mysterious. In the early twentieth century the traditional analytic scientific method of breaking things up into constituent parts to see how they worked began to look beyond the scale of individuals into larger groupings. In 1912 Alfred Wegener (1880–1930) published the theory of continental drift which gave impetus to more global interactions. Nineteenth century botanists like Joseph Hooker and Robert Brown had begun reasoned speculation on global plant distribution and this mode of thinking, supported by the work of people like von Humboldt, Alfred Russel Wallace and French-Italian Leon Croizat created a whole new interest in large-scale biological systems at a global scale known as biogeography.
At about the same time and at a slightly smaller scale scientists were looking more closely than ever before at the way plants and animals were interacting with one-another and their environment. This was the beginning of ecology which, by 1930,had produced the important ideas of plant and animal communities, succession, community change, food chains, energy flows and such which, from the 1940s, matured into and independent discipline as Eugene Odum (1913–2002) and others formulated many of the modern concepts of ecosystem ecology.
Genetics had provided a means to study the history and evolution of domesticated plants and the pioneering work on this subject by Frenchman Alphonse de Candolle was extended by Russian Nikolai Vavilov (1887–1943) who, from 1914 to 1940, published accounts of the geography, centres of origin, and evolutionary history of the economic plants that were now occupying so much of the earth’s surface.
Research is unending: every new discovery or problem solved gives rise to more questions. But if we were to bring back Theophrastus and tell him what we have found out since he and Aristotle were researching plants and animals on the island of Lesbos, I think he would be both amazed and deeply impressed. We now know how plants work: all basic questions concerning their structure and function have, in principle, been resolved. A World Flora has begun as an inventory of all the world’s flowering plants. Although we must acknowledge forerunners it was Theophrastus, more than others, who established the ‘initial conditions’ from which so much has flowed: ideas about plant collecting and redistribution, economic botany, the botanical garden as a place associated with education (and later university), and above all the legacy of Greek analytical empiricism that gave us science in general and plant science in particular.
What would probably have surprised Theophrastus more than anything else would be the scope of modern botanical knowledge as science’s application in technology has revealed the plant world at the micro and macro scales (see Reason & science).
So what problems are left to solve: what will plant science look like in years to come, and what are the problems yet to be resolved?
The fine detail revealed by microscopes of incredible sophistication, and chemical analysis that penetrates to the atoms first postulated by Democritus – the explanatory power opened up by genetics, understanding of the genetic code, and the molecular biology on which modern biotechnology rests.
Now the distinction between pure and applied botany becomes blurred as our historically accumulated botanical wisdom at all levels of plant organisation is needed (but especially at the molecular and global levels) to improve human custodianship of planet earth. The most urgent unanswered botanical questions now relate to the role of plants as primary producers in the global cycling of life’s basic ingredients: energy, carbon, hydrogen, oxygen, and nitrogen, and ways that our plant stewardship can help address the global environmental issues of resource management, conservation, human food security, biologically invasive organisms, carbon sequestration, climate change, and sustainability.