Beginning with the 16th century physiology became a topic of botany. The circulation of saps in plants, the transformation of soil compounds into plant-specific components, the connection of water uptake, sap pressure and evaporation and single plant substances became subjects of interest. During this time, too, botanical research began to employ physical and chemical methods.
The history of physiology is, just like that of systematics, made up of a long past history and a slow process of recognition. The knowledge of the 16th and 17th century did not differ from that of antiquity. It was known that the root did not only serve to keep the plant in the soil but, too, was necessary for the uptake of nutriments and that certain fertilizers like ash promoted growth.
C. PERRAULT (1613 - 1688) made extensive observations of the circulation of saps in plants. He believed the "fermentation of the soil humidity" and the effects of the saps in the roots to be the reason for the rising of the sap in the plant. His opinion was that warmth was created and that the sap would thus expand. He explained the descending movement of the saps by growth of the roots and an interaction between leaves and roots.
The prior of the monastery S. Martin Sous Traune, E. MARIOTTE (1620 - 1684), realized in 1679 that the most dissimilar plants can receive their nutriments from the same components of the soil and that they are able to form many more compounds than are to be found in the soil. In addition, he observed that one and the same sap can produce tangy tasting fruit in a wild pear tree and palatable ones in a fine scion that has been grafted to the wild tree. He proofed by destillation that the same species containes always the same compounds and thereby demonstrated that a transformation of matter must take place in plants. To confirm this view he made the following calculation:
"...... You can cultivate 3000 - 4000 different plant species in 7-8 pounds of soil.....Now: if saps, oils and minerals are different in each plant species, then all of these would have to be contained in this small quantity of soil and rain water. It is obvious that this is impossible. For if each of these plant species contained in a state of ripeness at least one gros fixed salt and two gros soil and all these added to those that are dissolved in water that would make up at least about 2-3 ounces in weight for each plant species, meaning that those of 4000 plant species would sum up to a weight of 500 pounds."
J. WOODWARD, professor at Gresham College in London (1665 - 1728), put a plant in a glass of water and covered the water surface so that evaporation was possible only by the plant. He discovered thus that the plant emits during three month 46 times the amount of water that it stores in itself. This typ of experiment was perfected by S. HALES (1677 - 1761), who also pointed out the importance of hydrostatics for the explanation of the rising of the sap. He determined a connection between the strength of the sap pressure and evaporation and introduced the scales as a new device for experiments, thereby establishing a link to physics. He tried, by comparing the weights, to determine the time period between water uptake and water evaporation and used his results to calculate the velocity of water transport through the plant.
The examination of single plant substances, especially that of acids, made progress through the work of the Swede C. W. SCHEELE (1742 - 1786), whose research into this subject began in 1770, and through that of the Frenchman VANQUELIN, who began to research at the end of the 18th century. They made tartaric acid, citric acid, malic acid, oxalic acid and gallic acid well-known substances. A. S. MARKGRAF from Berlin (1709 - 1782) and DUHAMEL du MONCEAU analyzed plant ashes and identified a number of salts. MARKGRAF is, too, the discoverer of cane sugar in sugar beet and was the first to use the microscope as an aid for analytic chemistry (sugar crystals in dried root sections). S. F. HERMSTAEDT (1760 - 1833; Berlin) introduced chemical analysis as a means of characterising useful plants. He emphasized the significance of fermentation and decay. The Swede J. J. BERZELIUS (1779 - 1848) listed during the first half of the 19th century already more than 40 different compounds isolated from plants.
In 1838 the Göttinger Akademie der Wissenschaften (The Academy of Sciences in Göttingen) organized a competition asking ".......whether the so-called inorganic elements, which can be found in plant ashes can still be found in plants, even if they have not been offered as nutriments and whether these elements are such important parts of the plant organism that it cannot grow properly without them."
The winner of the competition was the chemist JUSTUS v. LIEBIG (1803 - 1873) from Gießen with his work " Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie" (Organic Chemistry and Its Use in Agriculture and Physiology). He has to be appreciated for his determined use of chemical methods in the answering of questions on the nutrition of plants. v. LIEBIG examined the connection between the yield of crops and the amount of fertilizer given to them. He, too, recognised the importance of the mineral compounds made available to the plant by the decay of animal and vegetable material. When examining the nature of the soil compounds used by plants and the amount to which they are used by growing crop, he found out that the major elements needed by plants (C, H, O and N) are available in abundance so that supplying them was unnecessary. His results were completed by the analysis done by KARL SPRENGEL on the yield and nutrition of agricultural crops. SPRENGEL was able to prove that certain mineralic compounds are as important for the growth and prospering of plants in very small amounts as those compounds that had up till then in abundance been added to fertilizers.
Since the middle of the 19th century research especially that performed under the aspects of use and commercial interest was supported in ever increasing degrees. Independent research stations were founded, at first often privatly, later, towards the end of the 19th century and in the beginning of the 20th century also by state and industry. In Rothamsted near London the landowner LAWES founded a research station that was equipped with a large expenditure of money and extensive estates. It belongs till today to the outstanding, internationally respected agricultural research institutions: Rothamsted Experimental Station.
In Germany similar institutes were founded in some places. An interest in the significance of proteins from plants for human nutrition began to emerge and it was soon discovered that nitrogen was of importance to plant proteins while starch-containing compounds (carbohydrates) were devoid of nitrogen. Processes enabling scientists to determine the amount of soluble and insoluble carbohydrates and, independent of this, those of sugar and fat were developed.