Advances in Botany

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Advances in Botany

Overview

The eighteenth century saw the development of a new approach to the study of plants: an experimental approach. Botanists were influenced by the great strides that had been made in physics as a result of Isaac Newton's (1642-1727) work on finding the basic principles underlying the complexities of motion, and they wished similarly to find unifying concepts governing the structure and activity of plants. One way to do this was to study plant physiology, that is, to devise experiments that would tease apart particular aspects of plant function. By the end of the century great progress had been made in determining how plants transport water, use sunlight to produce oxygen, and rely on insects and birds in pollination (the process by which pollen, containing the male sex cell, is transported from one plant to another of the same species).

Background

While at the beginning of the seventeenth century the English philosopher Francis Bacon (1561-1626) had encouraged experimentation as a way to discover information about the natural world, it took some time for those interested in nature to develop experimental techniques, to devise ways to test their ideas about the world under carefully controlled conditions. One of the eighteenth-century investigators who made important contributions to this development and to botany was Stephen Hales (1677-1761). He is the founder of experimental plant physiology, although he also did significant research on animal blood pressure as well as important work on improving conditions in mines and on ships.

In 1727 Hales published Vegetable Staticks in which he reported on his studies of the movement of water in plants. At the time, some thought that water, in the form of sap, circulated through plants as blood circulates through animals. This was one of several such comparisons made during the eighteenth century between plant and animal physiology that were later found to be erroneous. Hales discovered that there was no pump comparable to the animal heart propelling water in plants, but that transpiration, the loss of water from the surface of leaves, caused more water to be drawn up into the leaves. There was also root pressure forcing water up into the stem from the roots.

Although the information he discovered about water movement in plants was important, what was even more significant about Hales's research is the way it was conducted. His work was not just descriptive, but quantitative. He took precise measurements to show not only that water evaporated from the leaves but the rate at which this occurred; to do this he covered tree branches with glass vessels in order to collect the water, and he worked with several different species to establish that this was a general phenomenon.

Hales also devised ways to measure plant growth. He drew equidistant marks on leaves, so the rate of expansion could be measured by the amount of displacement of the marks from each other over time. Though such techniques were simple, they had never been attempted before in botany. They led to many quantitative studies on plant growth, though it wasn't until the second half of the eighteenth century that significant progress beyond Hales's research took place.

Impact

Among the other areas of plant physiology he investigated, Hales studied the role of air in plant life, but he was hampered by the fact that the components of the air were not well understood at that time. He did make the important observation that air seemed involved in nourishing plants and that one component of the air was absorbed by the leaves. It was Jan Ingenhousz (1730-1799), a Dutch physician and botanist, who built on this work later in the century, publishing his Experiments upon Vegetables in 1779. A little earlier, the English chemist Joseph Priestley (1733-1804) had found that if he placed a mouse in a sealed container, it would quickly use up a component of air that was necessary for its life, though the rest of the air would remain. He also found that if a plant were then put into this container, it would sometimes restore to the air the component that the mouse had removed from it. Unfortunately, Priestley couldn't get consistent results; while this experiment sometimes worked, other times it didn't.

Ingenhousz used the techniques that Priestley developed to show that plants only restored the component of the air which the mouse used when they were exposed to light. He also showed that the generation of this component, which the French chemist Antoine-Laurent Lavoisier (1743-1794) named oxygen, didn't occur everywhere in the plant but only in the green parts, particularly the leaves. Finally, Ingenhousz demonstrated that plants absorbed the component of the air that the mouse had generated, called carbon dioxide, and that this gas was the source of carbon in plant material, not carbon in the soil. In other words, Ingenhousz worked out the basics of photosynthesis, the process by which plants use the Sun's energy to convert carbon dioxide and water into sugar and oxygen. At the beginning of the nineteenth century Nicholas de Saussure (1767-1845) built on Ingenhousz's work and showed that the amount of oxygen generated in photosynthesis was equal to the amount of carbon dioxide absorbed by a plant, indicating that the two gases were indeed involved in the same process.

Another area of botany that received a good deal of attention in the eighteenth century was plant reproduction. In 1694 Rudolf Camerarius (1665-1721) published a paper outlining his argument for the existence of sexual reproduction in plants. As was mentioned earlier, it was a popular idea at the time that there were similarities between plant and animal anatomy and physiology. While some attempts at showing likeness were misguided, Camerarius's work revealed that exploring similarities could sometimes be useful in understanding plants. He studied plant species such as mulberry that are called dioecious, meaning that there are two forms, one with stamens that produce pollen and one with pistils that produce seeds. He found that a mulberry plant with pistils won't produce any seeds unless there is a mulberry with stamens in the vicinity. He hypothesized that pollen grains were comparable to sperm in animals and were necessary for fertilization of eggs and their development into seeds within the pistils. Camerarius also studied the more common monoecious species, in which the stamen and the pistil are found on the same plant and often in the same flower structure, forming what is called a complete flower. By carefully removing the stamens from the flower of the castor bean, a monoecious species, he was able to prevent seed development, thus again showing the necessity of pollen for this process.

When Carolus Linnaeus (1707-1778) created his plant classification system in the mid-eighteenth century, he focused on the sexual organs found in flowers as the basis for his method. At the time of his research, significant advances were also being made on the work of Camerarius in terms of the mechanisms of fertilization. Joseph Gottlieb Kölreuter (1733-1806) dealt with a number of questions related to fertilization, using careful experimentation and observation. Camerarius had pointed out that sexuality in plants suggested that hybrids were likely, that is, that the pollen of one species could fertilize the egg of another, producing a plant with a mix of characteristics of both species. To investigate this question, Kölreuter developed a technique for artificial fertilization in plants. He removed the pollen-producing stamens from a Nicotiana rustica plant, and then brushed pollen from Nicotiana paniculata onto the pistil of the N. rustica plant. The hybrid offspring of this cross had distinct characteristics that were a mix of those of the two parent species. When the opposite cross was made—when pollen of N. rustica was applied to the pistil of N. paniculata—the offspring had the same characteristics as the offspring of the original cross, and these characteristics were stable, that is, always appearing when crosses were made between these two species. This suggested to Kölreuter that stable characteristics were at the basis of inheritance and that laws governing inheritance could therefore be discovered; in other words, a science of genetics was possible, though it would be a century before Gregor Mendel's (1822-1884) work on pea plants formed the basis for this science.

One important characteristic of the offspring of crosses between Nicotiana species was that they were sterile, that is, they could not reproduce. In later work on crosses between other species, Kölreuter found that most, though not all, of the hybrids were sterile, suggesting that sterility is what prevents such hybrids from becoming more common in nature. In the nineteenth century this finding became important to Charles Darwin's (1809-1882) theory of evolution because it helped to explain how the number of species increased over time: once new species came into existence, hybrid sterility helped to maintain their integrity.

Kölreuter also did microscopic studies on pollen, examining how the pollen adhered to the sticky stigma at the top of the egg-containing pistil. He disagreed with earlier botanists who had assumed that the pollen grains normally became swollen and burst. He correctly found that this only occurred when the grains absorbed abnormal amounts of water, and pollen normally didn't enlarge but instead sometimes grew an extension into the pistil. He didn't continue this research far enough to find, as later botanists did, that this extension, the pollen tube, carries the male sex cell down to the egg.

While Kölreuter made great strides using artificial fertilization techniques, he also did studies on natural fertilization. He found that few plant species were capable of self-fertilization in which pollen fertilizes an egg produced by the same plant; instead, pollen from a different plant of the same species was necessary for successful fertilization. This being the case, the question became how did pollen travel from one plant to another. Kölreuter's observations revealed that some pollen was wind-borne, but in other cases the pollen was carried from one plant to another by animals, most often by insects and birds. He also correctly hypothesized that the sugary liquid called nectar produced in some flowers was used to attract insect and bird pollinators. Kölreuter's long years of research contributed a great deal to botanists' understanding of fertilization, and this information was important in breeding studies that yielded many hybrids that became important food crops. Today, for example, most of the corn produced in the United States is hybrid corn and many of the most popular garden plants are also hybrids.

MAURA C. FLANNERY