Advances in Plant Classification and Morphology

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Advances in Plant Classification and Morphology

Overview

Beginning in the sixteenth century, as botanists explored more areas of the world and identified more and more species, the problem of how to best classify all these species became critical. By the mid-nineteenth century a number of botanists had devised classification schemes that were based on using a large number of traits. As the century progressed there was also greater interest not only in plants' external form or morphology, but also in internal structures and in microscopic examination of plant tissue. With this trend came more work on plant development, on how plant structures arose, enlarged, and changed over time. This increased morphological knowledge improved classification of plants by providing more information on which to base categorizing decisions.

Background

The basic classification system used by nineteenth-century botanists grew out of the work of botanist Carl Linnaeus (1707-1778) in the mid-eighteenth century. He developed a system for giving each species a two-part name, the first part being for the genus or group to which the species belonged and the second for the species itself. Linnaeus's classification system was called artificial in that he chose a particular characteristic as the basis of classification; in the case of many plants the flower was the structure used. All plants having similar flowers would be put into the same group even though this group would include plants that were otherwise very different from each other.

This system worked well in allowing botanists to identify plants, but to many botanists it seemed inadequate. A number of botanists, most notably Antoine-Laurent de Jussieu (1748-1836) in France, set out to create a more effective system at the end of the eighteenth century. These systems were called "natural" because they were not as arbitrary as that of Linnaeus; they were based on the examination of a large number of plant characteristics, not just those of the flower.

In the first half of the nineteenth century the greatest contributions to the study of plant classification were made by Swiss botanist Augustin de Candolle (1778-1841). He coined the term taxonomy to describe the theoretical study of classification—the investigation of the different ways to describe and categorize a species. Candolle's goal was to classify and organize information on all plant species. To achieve this goal he began work on what would become the 17-volume Prodromus, which provided classifications and descriptions of all known plants. Augustin de Candolle did not live to see this work completed. The last of the volumes were written by his son, Alphonse de Candolle (1806-1893), also a noted botanist.

In his studies on plant morphology Augustin de Candolle was influenced by the botanical work of the great German poet, Goethe, who was very interested in botany. Goethe (1749-1832) argued that there was a general plan or form that underlay all plant form, with particular species being variations on this general theme. Candolle, too, saw unity underlying the diversity of plant form. He also agreed with Goethe that the parts of the flower were all related to the leaf form—that, for example, petals and other flower parts could be seen as modified leaves. This idea was very influential in the nineteenth century and reflected an interest in finding a way to simplify or unify the study of plant form.

The diversity of plant species and even the great number of forms within a single plant—stems, leaves, and intricate flowers with many different parts—made the study of plants a complex business; any underlying simplicity would have been welcome. By the end of century, however, interest in this approach to morphology had waned because a unifying theme had come from another direction, not from the study of external morphology but from investigation of internal anatomy and development.

Impact

After the publication of Charles Darwin's Origin of Species in 1859, with its presentation of the theory of evolution or change in species over time, natural classification came to mean attempting to organize species in terms of their evolutionary relationships. Species that had evolved from a common ancestor, for example, would be grouped together. The theory of evolution thus provided a new concept on which to base classification, though it frequently proved difficult to figure out evolutionary relationships, and the idea of an entirely natural classification system remains an unfulfilled goal in biology.

One aspect of botany that contributed a great deal to evolutionary theory was the study of biogeography, that is, finding relationships between plant form and geographical characteristics such as climate and terrain. This field was first investigated at the beginning of the century by Alexander von Humboldt (1769-1859), a German geologist who spent several years exploring South America with botanist Aimé Bonpland (1773-1858). Humboldt was impressed with the way vegetation varied with altitude and with the amount of rainfall in a region.

Later, a number of botanists including Asa Gray (1810-1888) in the United States and Augustin de Candolle and his son Alphonse in Switzerland deepened this investigation. Alphonse de Candolle in particular studied the geographical distribution of a large number of plant species and came to the conclusion that most plants seem to have originated in one location. This provided support for the theory of evolution, since the theory includes the idea that the organisms most likely to survive are those that are best adapted to a particular environment. So it makes sense that plants in different kinds of climates would have different traits. Desert plants, for example, would have adaptations to prevent loss of moisture, while those growing on the shaded rain forest floor would be adapted to surviving in low-light conditions.

One criticism of many plant morphologists such as the Candolles was that they focused almost exclusively on what could be seen with the naked eye or with a simple magnifying glass. The microscope figured little in their studies, and, in fact, microscopic work was considered suspect by many botanists because they were uncomfortable with information obtained in this way; they were worried about what they were really seeing through the lenses of the microscope and how it was related to directly visible form. But as the century progressed, botanists became more comfortable with the use of this instrument, particularly as it became apparent that there was a complex world of structure at the microscopic level and that different botanists using different microscopes were seeing similar forms—in other words, that one botanist's results were reproducible by another, an important hallmark of scientific inquiry in general.

Use of the microscope led to a greater interest in plant development—in how the embryonic plant arose, grew larger, and became more complex. Much of the focus of morphological research had been on angiosperms, flowering plants, which are considered the most advanced plants and those that evolved most recently. But concern with development brought a broadening of botanists' investigations, with more work being done on non-flowering plants, (mosses, ferns, and gymnosperms, e.g. pines and fir trees).

British botanist Robert Brown (1773-1858) discovered that development in flowering plants began with the fertilization of the female cell or ovum by the male cell from a pollen grain; this process took place within a structure called the ovule at the base of the flower. Later, German botanist Wilhelm Hofmeister (1824-1877) found that the embryo then develops from the ovum and eventually becomes encased in protective layers to form the seed. Hofmeister was one of the leading plant morphologists of the nineteenth century. He studied development not only in flowering plants but in a variety of other plants including ferns, and this is how he came to discover the great unifying principle underlying plant form.

Ferns had been studied by a number of noted botanists before Hofmeister; they were a fascinating group of plants because they were quite different from flowering plants, producing no flowers or seeds but rather spores that lacked the protective coatings of seeds. The spore develops not into a mature plant, but into a tiny, leaf-like structure called a prothallus or gametophyte. German botanist Carl von Nägeli discovered a set of cells within the gametophyte called the antheridium that produced sperm and was thus comparable to the pollen-producing anther of flowering plants. It was Polish Count J. Leszczyc-Suminski who later found the archegonium, the structure in the gametophyte that produces eggs. Fertilization takes place on the gametophyte and a new structure, the sporophyte, develops on it. It is the sporophyte that eventually grows into the mature fern plant on whose leaves or fronds arise the sporangium, the spore-producing structures.

Hofmeister saw the fern as having two parts to its life-cycle, with one part, the gametophyte, producing sex cells, eggs and sperm, and the other producing the sporophyte or mature plant. We now know that the gametophyte is haploid, that is, it has half the number of chromosomes as the diploid sporophyte, but Hofmeister made his observations before the role of the chromosomes as carriers of genetic material was understood. He did see, however, that this alternation of generations, between gametophyte and sporophyte, between haploid and diploid structures, was found in all plants, not just in ferns, though the size and structure of the two forms varied widely.

In flowering plants the sporophyte structure is clearly dominant, with the pollen being the male gametophyte and the embryo sac found within the ovule at the base of the flower being the female gametophyte. The sporophyte is also dominant in gymnosperms such as pines, but in seaweed or green algae the gametophytes and sporophytes are often so similar in size and structure as to be indistinguishable from each other when examined with the naked eye. In moss, the visible form of the plant is the gametophyte, with the sporophyte usually forming small structures arising from the gametophytes. Such wide variation in structures makes Hofmeister's discovery that much more impressive, and it provided the first really significant unifying principle in botany, an idea that related the structure and development of all plants.

MAURA C. FLANNERY