Paper presented at the Australasian Science Education Research Association, Darwin,
Australia, 9-12 July 1998
Miles Barker
University of Waikato, Hamilton, New Zealand
Histories today often imply that questions about the movement of fluids in plants were initially addressed, and also decisively resolved, by the meticulous experiments of Stephen Hales in eighteen century England. In fact, in demonstrating that there is a through-flow of water in plants (which we now call transpiration), Hales also had to refute the longstanding view that sap circulates. As well, a profound methodological switch was involved: Hales's successful application of experimentation in plant physiology required him to abandon the traditional approach - the search for anatomical analogies between plant conducting tissues and blood circulatory systems in animals - which can be traced back to the beliefs of Aristotle. Current textbook histories of science which document only Hales's pivotal experiments appear to promote the traditional, often unsuccessful classroom discovery strategies frequently in use today. However, an account of science history which reveals to teachers the exuberant richness and variety of scientists' investigations, theories and methods appears better able to interpret students' everyday ideas and the language they use, to suggest innovative teaching strategies, and to promote learning which is rich and complex.
From the standpoint of today's knowledge about plant transpiration - the upward movement of water as xylem sap from roots into the shoot system, and its loss from leaves by evaporation (Campbell 1996) - the theory that plants may possess a circulatory system comparable with that of animals may seem eccentric, if not down-right absurd. However, the history of biology shows that in the seventeenth century this kind of argument by analogy appeared to be a promising approach to understanding plant physiology. For example, in 1673 English botanist Martin Lister (1639-1712), inspired by William Harvey's account of blood circulation in animals, published forty-five years previously, was among the first to speculate that a totally analogous system might exist in plants. While admitting that botanists had "not yet here discover(ed) any uniting of veins into one common trunk, no pulsation, no sensible stop by ligature, no difference in veins, etc", Lister nevertheless hoped that this lack of anatomical justification would "in time be happily overcome; and the analogy betwixt plants and animals be in all things else, as well as the motion of their juice, fully cleared" (Lister 1673).
Lister's approach to explaining "the motion of their juice" was not an isolated insight. The search for animal-based analogies for plant functions was at least as old as the work of Aristotle, and in Lister's time this approach was still actively applied to questions of plant nutrition and sexual reproduction. This paper traces the development of the "analogist" (Delaporte 1982) tradition, and how, with the meticulous quantitative demonstration by Stephen Hales (1677-1761) that water movement in plants is noncirculatory, this was succeeded by an "experimentalist" tradition. The paper concludes with a discussion of some of the implications for learners today.
The "Analogist" Tradition
Although he was steeped in the traditions of Aristotle, the Italian naturalist Cesalpino (1519-1603) has also been described (Harre 1970) as the first modern plant physiologist. Cesalpino provided one of the earliest detailed accounts of the movement of sap - he postulated that plants draw a nutrient sap into the plant from the damp soil by the sponge-like dry roots, and convey it up into the stem through tiny veins (Sachs 1890). These findings, the outcome of Cesalpino's personal engagement in collecting, examining, describing and growing plants (Morton 1981) reflected a slight but significant shift away from the Aristotelian framework within which he worked.
Aristotle (c.384-322 BC) had considered that all organisms were arranged in a scale of being, marked by increasing structural and functional complexity. Absorption of food was common to both plants and animals (Barker 1995) but other functions, like sexual reproduction, were carried out only by animals (Barker 1998a). Cesalpino echoed Aristotle's view that plants, like animals, absorb their food but his findings also reflected a growing tendency to widen the process of analogy-seeking beyond matters of nutrition. For example, he declared that figs and vines, when cut, bleed like the flesh of animals, and that plants therefore also probably have veins, although they are only very narrow, in accordance with the smaller mass of food which plants need (Sachs 1890).
There is no hint in Cesalpino's ideas of a circulation of sap. The absence of a heart (which was held to be the laboratory of the warmth of the animal body) was enigmatic, but was at least consistent with the lower temperature of plants. The discovery by William Harvey (1578-1657) of the action of the heart and the general blood circulation in animals was enormously significant for the botanists. Johann Daniel Major (1634-1693) of Kiel claimed to be the first to publish, in 1665, a circulatory theory. Major argued that sap must circulate because plants could not grow "if the motion of sap were only from bottom to top, that is from the root to the top of the stalk, and if it never returned by nervous or fibrous channels analogous to the veins of animals" (Major 1665).
In England, among members of the Royal Society, speculation was also rife. The espousal of circulation by Martin Lister in 1673, quoted above, had emerged from observations (Ray and Willughby 1669) which suggested to Lister that fluids move in plants - as in animals - all year round; and from his own view that the fluids are confined to "vessels analogous to our human veins, and not mere pores" (Lister 1672). Lister's views would have found favour with Richard Reed, who had declared that it was "an heresy in husbandry obstinately to deny the descent of sap" (Reed 1671). On the other hand, John Beale had considered that "the main quantity of sap" moved upward only and "is gradually hardened into leaves, blossoms, fruit, timber in such manner as the ossification in young animals" (Beale 1671).
The "Experimentalist" Tradition
In the mean time, debates over the function of leaves were turning research in new directions. Marcello Malpighi (1628-1694), who realised that leaves somehow elaborate simple materials into the complex stuff of plants, reasoned that there must be a return, downwards movement of sap. However, this did not have the regular rhythmic character of blood circulation but was a slower, more uneven process, subject to seasonal change (Harre 1970). Similarly, the realisation by Nehemiah Grew (1641-1712) that the atmosphere must contribute food material to plants was probably ultimately more decisive than the beautiful, but sometimes mystifying microscopic observations of ramifying systems of tubes which he and Malpighi made. For example, Richard Bradley concluded that "these tubes are not large enough to admit into them any thing more gross than vapour" (Bradley 1716).
In his Vegetable Staticks of 1727 Stephen Hales (1677-1761) envisaged plants as "very probably drawing through their leaves some part of their nourishment from the air" (Hales 1727) and hence he further advanced the notion of leaves as centres for nutrition. However, his decisive rebuttal of the sap circulation theory came from his experiments on water relationships, i.e. transpiration. Hales showed that huge quantities of fluid are absorbed by roots. (He calculated, for example, that seventeen times more water enters "heartless vegetables" bulk for bulk than a man.) This tallied with the water given off by leaves, and could only be accounted for by "a progressive and not a circulatory motion, as in animals". Further evidence against circulation came from experiments involving ringing the bark - Hales showed that "the bleeding of tree above that bared place will much abate, which ought to have the contrary affect ... if the sap descended". Again, although he discovered clear mechanisms to explain upward motion, i.e. "the strong attraction of capillary sap vessels, assisted by the brisk undulations and vibrations, caused by the sun's warmth", Hales found it "hard to conceive what and where that power is" to promote downwards movement. Finally, he pointed out that it was not necessary to postulate a circulatory system to accommodate the possibility that at night "the body and branches of the vegetable which have been much exhausted by the great evaporation of the day may imbibe sap and dew from the leaves" - Hales considered that at night "sap in all vegetables does probably recede in some measure from the tops of branches", rather than being transported away by a form of circulation.
Hales's work was decisive - "his reasons have been thought so convincing, that the system of circulation in plants has been ever since exploded in England; and ... they have a similar effect abroad" (Maty 1773) - and it provided the basis of today's accounts of the upward xylem transport of water and minerals. One important error was Hales's suggestion that leaves sometimes absorb significant liquid water, although this possibility had been voiced at least from the time of Leonardo da Vinci, who noted that "every branch and every fruit comes above the insertion of its leaf, which serves it as a mother by bringing it the water of the rains and the moisture of the dew that falls there at nights" (MacCurdy 1956). Incidentally, Hales's view that the sap might "probably recede" at night - to accommodate his postulated leaf absorption - was also later shown to be a misconception.
In the hundred years following Hales's experiments, his successors added little to the science of plant physiology (Harre 1981) and it was not until 1914 that the Irish botanist Henry Dixon (1869-1953) resolved the debate over the mechanism of xylem transport by proposing a compromise between the pulling power of the leaf and the tensile strength of the water columns (Bynum et al. 1981). Present day accounts based on Dixon's work (for example, Salisbury and Ross 1992) attribute the ascent of sap to three factors: a driving force in the leaves due to solar energy evaporating water into the air where a much lower water potential exists; cohesion forces between water molecules made possible because of the capillary dimensions of the xylem elements in the stem; and the transference of negative water potential to the root cells and soil which draws water into the root hair cells.
The significance of the phloem, as the conducting tissue for organic assimilates produced in the leaves, also emerged only slowly from the conflicting studies of Theodor Hartig (1805-1880) and Julius Sachs (1832-1897) and others in the nineteenth century. In 1926 Ernst Munch (1876-1946) proposed the pressure-flow model (favoured by most plant physiologists today) to describe the slow, seasonal reversals of materials in the phloem (Salisbury and Ross 1992). By Munch's time, the phloem was known to be physically discontinuous with the xylem, but even today there is a sense in which a tiny amount of water can be said to circulate - experiments with tritiated water (Salisbury and Ross 1992) show that some of the 1% of non-evaporated water makes it way across leaf tissues from xylem to phloem.
Portraying Science History Today
This account of the replacement of the "analogist" tradition by the "experimentalist" tradition in the search for explanations about plant sap movement suggests three interesting implications for the way the history of science should be portrayed in educational settings today:
1. Embedding "Key" Experiments in their Context
In mentioning Stephen Hales, today's text-books usually fail to convey the exuberant richness and diversity of his experimentation. For example, Salisbury and Ross's (1992) Plant Physiology, a foundation tertiary text, cites only Hales's (apparently decisive) "girdling" experiment. Here, Hales removed a strip of bark around a trunk, found that the bark above the girdle swelled and that below shrank, and concluded (correctly) that transpiration must occur in the underlying wood and that nutrients essential to the life of the bark (and presumably the roots) must move in the bark. However, even a cursory reading of Vegetable Staticks suggests that, for Hales, no single experiment was compellingly persuasive - his theory of transpiration is clearly a mental model constructed from the outcome of dozens of experiments.
2. Taking Account of Received Theories
Texts tend to imply that Hales formulated the problem of sap movement and its mechanism, and that he was working in the absence of a received theory. However, a more careful reading of Vegetable Staticks shows that Hales was very conscious of a need to refute the analogists' view, and that a significant part of the book is devoted to citing experimental evidence to achieve this. Viewed this way, Hales was not only proposing novel experimental findings - he was confronting a tradition which can be traced back to the vast metaphysical scheme which Aristotle proposed. His notion of the "scale of being" (see above) was underpinned by a concept of a tripartite psyche - a kind of functional organisation (Putman 1994). Insofar as the psyche was an Unmoved Mover, one of the fundamental Aristotelian physical principles (French 1994), it was linked to Aristotle's notions of the four terrestrial elements (Cohen 1996) and hence to his cosmology at large (Burke 1985).
3. Exploring the Constraints and Possibilities of Method
Whilst Harvey's circulation theory of 1628 strongly influenced the botanists, it is also necessary to remember that De Motu Cordis was also "a model for experimental research" (Goodman 1965). Why did it take so long before anyone relinquished analogy and applied the experimental approach to the sap question? That it was Hales appears to have resulted from the latter's contact at Cambridge University with Isaac Newton and his chemical philosophy (Guerlac 1951). Newton's opening of the Opticks, published in 1704, implies that the work is admirably free from the conjecture, analogy, metaphor or hyperbole - approaches that, in the seventeenth century, were increasingly unfavourably compared with unadorned objective knowledge (Lakoff and Johnson 1980, Holton 1993). Although somewhat reluctant to relinquish analogy (Bynum et al. 1981) Hales included fifteen quotations or references to the Opticks in Vegetable Staticks (Cohen 1952) to justify his application of Newton's Achymo-statical experiments@ to the study of plants. Subscribing to a new heuristic appears to have been the key which unlocked the potential in Hales's technical ingenuity.
How History Can Inform Science Learning
Such a portrayal of science history, which takes due account of its contexts, received theories and methods, can point out ways to a more informed, enriched and successful classroom practice. The findings of a survey (Barker 1998b) of students' understandings of transpiration, after traditional teaching, were analysed in terms of the relevant history.
The survey comprised individual interviews about plants and water uptake, conducted with sixty 8-17-year old students, thirty-six before traditional teaching about transpiration and twenty-four after. The teaching comprised examination of stained stem sections, viewing stomata, collecting water vapour from leaves in plastic bags and demonstrating its presence with cobalt chloride paper and/or anhydrous copper sulphate, and having the teacher construct a summary diagram which depicted the upward passage of water through stems from soil to air. The study revealed that 25% and 22% respectively considered that leaves (as well as roots) absorb water, and 56% and 37% respectively believed that plants retain all of the water which they absorb.
Taking account of the history of learning about plants and water has helped to problematise existing ways of teaching about transpiration, and to suggest innovations which reflect the contexts in which children perceive classroom experiments, the received theories they apply, and their assumptions about what methods are appropriate.
Just as no single experiment was decisive in Hales's understanding, so is it unlikely that classroom reliance on the few customary experiments (potometers and so on) will disabuse all students of their preconceptions about plants and water. The pedagogical defects of such discovery approaches (Atkinson and Delamont 1977, Wellington 1981) and the ethical dilemmas they can present for teachers (Nott and Smith 1995) are well known. In this case, pitched against accepting the experimental evidence which teachers present (potometers, chobalt chloride paper, and so on) are, and always have been
How might these insights be transformed into practical classroom activities? Some suggestions are:
Conclusion
This case study has suggested that history of science, when viewed by educators in its richness and complexity, can encourage us to esteem and seek out the apparently implausible in our students, it can encourage us to problematise existing teaching practice, to be playful and student-centred in our design of classroom experiences, and it can promote learning which is similarly rich and complex.
I am very grateful to the following people for helping me to locate archival material: Anita Karg (the Hunt Institute, Carnegie-Mellon University, Pittsburgh), Susan Fraser (New York Botanical Gardens), Mike Gollop (the Brotherton Library, University of Leeds), and Dr Reinders Duit (Institute for Science Education at the University of Kiel). Dr Charlotte Wallace and Rosemary de Luca of the University of Waikato are also to be thanked for their thoughtful input to this paper.
Correspondence: Miles Barker, Senior Lecturer in Science Education,
Department of Mathematics, Science, Communication and Technology Education, School of
Education, University of Waikato, Private Bag 3105, Hamilton, New Zealand.
Internet email: mbarker@waikato.ac.nz
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