Development of Plant Taxonomy
|Internal Links: Beginnings; Post printing; Developments; Herbalists; Linnaeus; Natural Systems; Quotes; Readings.|
All cultures, whether based
on hunting and gathering or advanced technology, place plants in groups
and give names to those groups. Those cultures that are most directly
connected to their raw plant materials tend to recognize such functional
groups as food plants, poisonous plants, medicinal plants, plants that
are useful for building shelters, and those that make good weapons.
Within each of these groups, plants that look alike are apt to have
A Greek, Theophrastus (370-285 BC) is the first person in the European tradition of plant taxonomy. He was concerned with identifying the essential features of plants - what aspect consistently differentiated one plant from another. To appreciate what an enormous question this was, and how great Theophrastus' contribution, read The naming of names by Anna Pavord (2005; Bloomsberry Press). Where Theophrastus stood out from his successors was his reliance on observation and scepticism concerning reports by others.
Theophrastus grouped plants on the basis of their habit and duration, recognizing trees, shrubs, trees, annual, biennial, and perennial herbs. He must have been a keen observer because, in his book Historia Plantarum, he described 480 kinds of plant, recognizing the difference between inflorescences in which flowering occurred from the outside inward as different from those in which it occurred in the reverse direction (i.e., centripetal versus centrifugal flowering), differences in ovary position, and in corollas with united petals versus those with separate petals. These seem minor items now, but to recognize them as somehow significant when everyone else simply says plant a is different from plant b is not easy. He did not appreciate how important flowers were to plants. He knew that shaking what we would now call male date flowers over female date flowers resulted in more fruit, but did not appreciate the significance of the pollen. Then again, how much did people really know about what happens during sex in animals at that time? If you have any interest in the history of ideas, read Pavord's book.
Albert Magnus appears to have been the first to notice the difference between monocots and dicots; he lived around 1200 A.D. Magnus even noticed differences in stem structure, a pretty incredible feat when you consider that your 10x hand lens is probably as good or better than any he had available.
There was not a lot of progress in plant taxonomy during the next 400 years, particularly not in the grouping aspect. Knowing about plants fell in the realm of medicine, the chief interest of people being either in plants as food, medicine, poisons, or aphrodisiacs. There are relatively few plants that one can eat and these were soon known; fewer people knew which ones were medically important. Several herbals were published; they were often little more than copies of each other.
When printing became available in the early 1400s, the first botanical books printed were herbals. In these, the emphasis was on descriptions and accounts of the medicinal and domestic value of individual species. Some of these had a few beautiful illustrations; others had illustrations that owed a lot to the artist's imagination or faulty memory. All were enormously expensive by today's standards, but much cheaper than before the invention of printing.
Some herbalists contributed to the development of taxonomy as we understand it. For instance, Brunfels (1464-1534) distinguished between plants that had flowers and those that did not. Brock (1489-1554) (also known as Hieronymous) grouped the plants in his herbal according to their growth habit (i.e., as trees, shrubs, and herbs), but within each of these categories he "placed together, yet kept distinct, all plants which are related and connected, or otherwise resemble one another ". His herbal is also noteworthy for three other points. First, unlike most of his predecessors, Brock based his descriptions largely on his own observations; his predecessors relied heavily on descriptions of others, often going back as far as Theophrastus, Dioscorides, and Pliny -after all, everyone knew that the Greeks and Romans were the source of all knowledge. The other distinguishing features of Brock's herbal were 1) its incorporation of distribution information and 2) his debunking of much of the folklore concerning the supposed virtues or evils of particular plants.
At this time, it was not impossible for someone intrigued by plants to learn the names of all (or almost all) those with which he or she would come into contact, certainly all those that were in general use in Europe, probably around 1000 or so. (Just to give you some perspective on this: Flora Europaea, which was published in 1979, includes 11,577 species, many of which are introductions; the second edition of A Utah Flora, published in 1993, includes 3,284 species, 682 being introductions.
So long as Europe remained, in effect, isolated, there was no need for any systematic means of grouping or identifying plants; all one needed was a good memory. This was more or less the situation in 1500 A.D., but things were changing. It is convenient to regard publication of Linnaeus' Species Plantarum in 1753 as the end of the first stage in plant taxonomy or the beginning of the second but, before examining the reason for this, consider what was going on in the world between 1200 A.D. (Albert Magnus) and 1753 A.D. Five hundred and fifty years seems like a very long time to us, but bear in mind that change came more slowly before television, telephone, newspapers, trains, planes, and the internet.
Marco Polo made his voyage to China in the thirteenth century (i.e., the 1200s). There had been trade with China prior to his trip, but the trade had been carried out through intermediaries; Europeans did not get far beyond the western Asia, i.e., Palestine (including what is now called Israel), the destination for the Crusaders. Consequently, Marco Polo's trip was truly amazing. He came back with tales of great wonders and great wealth. It is worth noting that the Arabs and other Muslims were more widely traveled than Europeans, who were most Christians, at that time. Many Muslims made the pilgrimage to Mecca where they would have come in contact with people from many different countries. Moreover, most traveled by land. Consquently, there were well known land routes to China and western Asian from North Africa.- whereas Europeans tended to think in terms of traveling by boat.
In 1453, Constantinople was captured by the Ottoman Empire (it had previously been held by the eastern branch of the Holy Roman Empire). The capture of Constantinople made anything originating in Asia or India very much more expensive for Europaeans because this meant almost the whole route to the east was under the control of the Ottoman Empire which demanded ever increasing tolls (charging what the traffic would bear). And there were strong memories of the Crusades that did not endear Europeans to the Otttomans, or vice versa. Sound familiar?
Prince Henry of Portugal pursued one possible answer to these higher prices: get to the East by going south from Portugal and around Africa. This took a lot of courage; it was unknown territory and there were lots of horror stories about what would happen to anyone that tried to do it. Nor was it obvious that one could go round Africa. The first Portuguese ship to sail past the equator did so in 1471. To the surprise of many, the sailors lived to tell the story. Fifteen years later, Bartholomew Diaz became the first European to round the Cape of Good Hope, but he did not go very far past it because his crew threatened to mutiny.
Not until 1498 was it clear that one could go round Africa to what, from Europe, is the Far East. In that year, Vasco da Gama rounded the Cape of Good Hope and landed in Calcutta, a city on the west coast of India. As a reward for opening up this trade route to the east, Vasco da Gama was awarded a large pension and the Portuguese title of 'Dom'. As some of you have probably realized, Columbus had discovered the West Indies six years earlier. Unfortunately for Columbus, his discovery was not financially rewarding; he died in poverty.
Once Columbus and da Gama had shown the way, several more expeditions went out, gradually exploring more of both the New and Old Worlds. Another landmark date is 1517. In that year, Luther nailed his 95 arguments against indulgences to the door of the cathedral in Wittenberg, challenging the pope. This was not the first time that the pope's authority or the teaching of the Roman Catholic church had been challenged in Europe, but this time the challenge stuck. People left the church and did not get killed by lightening bolts (although many were killed in the subsequent religions wars).
What has all this to do with plant taxonomy? First, although the impetus for the voyages of exploration was a mixture of commerce and religion, they also lead to more and more different plants (and animals) being brought to Europe or described (sometimes rather imaginatively) by those who went on the ships and it became harder and harder to for anyone to know all the different kinds of plants and animals. Some sort of system had to be developed 1) for placing the new plants in groups, whether new groups or old groups, and 2) for naming the groups.
A factor favoring the development of such a system was that people were becoming bold enough to challenge old ways, old authorities. Knowledge of the classics, and ability to speak Latin and Greek were still essential for anyone wishing to be considered educated, but people were no longer so ready to believe a thing true simply because the church or a one of the classical scholars had said it.
Let us be honest here, 'people' in the preceding sentence refers to the small proportion of the population that was educated, but this proportion had grown enormously since the introduction of printing. In the 1500s it began to be expected that any well to do person should be able to read and write, not just priests and merchants. Moreover, it became more acceptable for intellectuals to devote their energy to consideration of the natural world rather than church doctrine.
The rapid increase in the number of plants known is reflected in the major herbals produced during this period. In 1583, Andrea Cesalpino, an Italian, wrote De Plantis (On Plants) in which he described about 1500 plants. He also outlined a basis for classifying, or grouping, plants, but it was not taken up.
In 1650, Jean Bauhin, a French and Swiss physician, published the Historia plantarum universalis (Comprehensive Treatment of Plants) in which he treated 5000 species. Bauhin's work is also notable for 1) including the first good species descriptions, 2) distinguishing nomenclaturally between species and genera, and 3) using a binomial nomenclature.
In 1703, a Brit, John Ray, published Methodus plantarum (Plant Systematics) in which he classified nearly 18,000 species according to their form and gross morphology. This represents a 10-fold increase in 120 years! Ray's groups are generally considered superior to Linnaeus', but there are good reasons why Ray's is considered less important than Linnaeus (as Linnaeus would have been the first to point out).
So, what did Linnaeus do? Linnaeus provided an easy method of classifying (grouping) plants and, somewhat incidentally, a system of nomenclature. Linnaeus' grouping system was simple; all you needed to do to put a flowering plant in its correct group was count the number of different kinds of parts, and the number of stamens and pistils, and see how they were arranged [i.e., always in the same flower, sometimes in different flowers, always in different flowers, always in different plants, etc]. He even prepared what were, in effect, keys to his major groups.
After identifying the group to which a plant belonged, it was necessary to read each generic description through and see which one best matched your plant and then repeat this process to determine which species in the genus it belonged to. This sounds so simple, but no one had done it before, and no one did it for so many plants (and animals) as Linnaeus, as he himself pointed out. Another distinguishing feature of Linnaeus' work was the consistency of his descriptions, all of which were based on his own observations.
Linnaeus' system was an immense success because it made it possible for any botanist to place a new plant into the appropriate group relatively easily, whether or not he (or she, but I do not known of any women botanists from Linnaeusí time) had ever seen any other members of that group or not. It did not necessarily place closely related genera or species together; Linnaeus knew this, but that was not his aim. His aim was to provide an easy to use classification scheme, one that would enable everyone to sort things quickly and consistently. In this he succeeded, admirably.
Our present binomial system of botanical nomenclature also stems from Linnaeus, but it was not part of Linnaeus' aim. In his day, the name of a species species one was considered to be a generic name followed by a phrase that described which species of the genus one was referring to. (In Latin, adjectives usually follow nouns; Catholics are familiar with this structure in such phrases as "Mary -full of grace"). For instance, one plant was named "The Pontederia with cordate leaves and flowers in a spike". But this takes a long time to say and write so, for each species he described, Linnaeus put a single word in the margin, a different word for each species in a genus. Our scientific names are obtained by combining the generic name with this marginal name, The Pontederia with cordate leaves and flowers in a spike had the adjective "cordata" against it; we now refer to it as Pontederia cordata.
Linnaeus was not the first person to use binomials, but he was the first person to provide a binomial for every species he described, and he described a lot. Moreover, in his teaching, he routinely used the binomials to refer to the species, not the formal names. For this reason, Species Plantarum is taken as the starting point for botanical nomenclature. You will never see Bauhin or Ray as the authority for a scientific name. They used binomials before Linnaeus, but not so consistently nor for such a large number of plants.
Linnaeus' system was used for a long time in many parts of the world. One such botanist was Willdenow (1765-1812; his name appears as Willd. as an authority for many species ), who edited the last edition of Species plantarum and was Director of the Berlin Botanical Gardens. Notable Americans employing the system were Schweinitz, Muhlenberg (Muhl.), Pursh, Walter, and Eaton. By 1820 or so, most taxonomists were no longer using it, preferring more 'natural' classifications (see below), but even in the early twentieth century a guide to the Flora of India was published that used the Linnaean system, not because it was thought to reflect relationships but because it was a handy method for making a 'first sort' of poorly known plants. The analogy that comes to mind is cleaning up a room that is in a total mess. Sometimes it is best to put everything away somewhere first so that one has enough space to think through how one really wants to organize things in cupboards etc. Linnaeus gave us a first sort and some rules that enabled everyone else to do a first sort in the same manner. The next step was to develop a classification system that was more natural.
Before leaving Linnaeus, he should be given credit for two other contributions. One was his ability to inspire others with his enthusiasm for getting to know as many different plants as possible. During his heyday, he had around 70 or so students in his classes. The second contribution was setting the precedent for including a botanist on voyages of exploration. These botanists were his own students, most of whom initially sent their specimens back to him, but many went on to become well-known botanists in their own right. Peter Kalm was sent to America by the Universities of Abo and Uppsala. Hasselquist and Forskal sent specimens from Arabia, Syria, and Egypt. Thunberg sent plants from the Cape of Good Hope and subsequently achieved a real coup, a post as surgeon in residence at a port in Japan. This made him the first European to collect extensively in Japan. In a very real way, Darwin's inclusion on the Beagle was part of the tradition started by Linnaeus.
French taxonomists were the only group that never used Linnaeus's system, sticking to that prepared by Tournefort, a Frenchman, until 1789, the year of the French Revolution and the year of publication of Genera Plantarum by the last in a family of French taxonomists, Antoine Laurent de Jussieu.
To summarize: Linnaeus developed a logical system for assigning plants to groups, provided a major impetus for the universal use of a binomial system of nomenclature, inspired a large number of and students, persuaded those sponsoring exploring expeditions to include naturalists as an integral part of the expedition.
The next steps in taxonomy involved a) to improving on the groups recognized and b) formalizing the binomial system of nomenclature (naming). Improving on Linnaeus groups meant circumscribing [circumscribing = describing, with an emphasis on the boundaries of] more natural groups. Linnaeus himself recognized this need and published a book, Systema Naturae, in which he outlined such a system. His natural groups were not, however, as easy to recognize as his artificial groups, nor was it as easy to determine to which natural group a plant belonged.
What is meant by a natural system? There has been quite a bit of argument about this, but basically it means that the groups recognized should reflect the relationships among plants; basically, this has, in the past, usually translated into placing plants with similar morphology [= shape, appearance] together. Remember, we are talking about the late 1700s and early 1800s. Darwin had not written his book on evolution and the mechanisms by which characteristics were passed on from one generation to another were unknown. It was recognized that offspring take features from both their parents and that hybrids are usually sterile, but thinking about the origin of species etc. was rather fuzzy. Biologists and geologists were becoming unhappy with the idea of a single creation event, but there was a lot to find out before the concept of descent by evolution could be articulated.
Among the discoveries made in the period 1753-1853 that contributed to a better understanding of plants were a) improvement in the understanding of optics and the consequent development of better microscopes, b) an awareness of the biological significance of the stamens and pistil in plants, and c) a better understanding of floral morphology.
We shall come back to the topic of how to form taxonomic groups, but from a conceptual rather than historical point of view. First, for practical reasons, we need to distinguish between three much used (and abused) words in the context of plant taxonomy. These words are recognition, identification, and classification. Before going there, consider a couple of quotes:
Letter to Linnaeus from
Collinson, written in 1753:
Camp, in a paper on generic
concepts (Bull. Torrey Bot. Club 67:381-389 (1940):
Judd et al. 2008. Chapter 3. More emphasis on approaches - worth reading.
Barkley, T.M. 1986. History of plant taxonomy. Pp. 37-59 in A.E. Radford, Fundamentals of Plant Systematics. Harper & Row, New York.
Bensen, L. 1979. Historical development of classification systems, Pp. 523-532 in L. Benson, Plant Classification. D.C. Heath and Company, Lexington.
Jones, S.B. and A.E. Luchsinger. 1979. Historical background of classification. Pp. 7-26 in S.B. Jones and A.E. Luchsinger, Plant Systematics. McGraw-Hili, New York.
Pavord, A. 2005. The naming of names. Bloomsbury Press, New York. By far the best book I have come across on the development of taxonomic thought.
Albert Magnus, 1200-1280 AD; Dominican Monk. Recognized that palms grow by stakes rather than in circular layers.Like Theophrastus, an observer and thinker.
1500s First botanic gardens
Ghini, a professor at Pisa, came up with the idea of a hortus siccus, or dry garden, what we call a herbarium collection.