Leonardo now had the opportunity to follow up an early interest—the study of plants. He made many beautiful drawings; no plant was too small to catch his eye. His notes on botany began to grow. With his genius for observation and analysis of nature, Leonardo made some extraordinary discoveries of botanical laws entirely unknown before his time. He wrote of the phenomenon of _heliotropism_, or the movement of plants toward or away from the sunlight. In addition, he described the phenomenon known as _geotropism_, or the growth of plants according to gravitational law, as for example, roots growing downward and shoots growing upward. He also defined the laws of phyllotaxis, which describe the system or order of leaf arrangement on a plant’s stem. That is, leaves are arranged spirally around a stem so that the third leaf above grows out over the third leaf below on one type of plant; or, on another type, the two third leaves are over the two third leaves below. The same natural laws apply to the branches of plants as well; they occur so that every leaf and branch can receive sufficient air and light. Amazingly enough, these laws, which Leonardo described so completely, were not rediscovered until almost two centuries later! Leonardo went even further in his botanical studies. He experimented with gourds, planting them in various aqueous solutions; this anticipated modern methods of growing plants in chemicals. He also tested the actions of arsenic and mercury poisons in plants. He reproduced the shape and form of leaves by pressing them on paper coated with lampblack, a method that was not used again until the nineteenth century. Carefully noted, too, in his writings was the rising of sap from the roots to the branches by capillary action; this, too, was not rediscovered until much later—in the eighteenth century. Leonardo also extracted oils and essences from flowers and studied the influences of altitude on the development of vegetation. Indeed Leonardo’s very approaches to a systematic classification of plants were the forerunners of modern methods of classifying.