How Plants Grow and How They Are Nourished
Mariotte and Tournefort explained plant growth mechanically, Homberg chemically. Mariotte thought that as water evaporated, various "earthy, salty, and oily parts" remained to mix and unite with the plant, creating "the hardness and solidity of the branches." He argued that although plants could not select what they took from the soil, they could transform it and actually absorb more oil as they matured. For mechanists like Mariotte and Tournefort, the circulation of the sap caused growth by putting pressure on the extremities of the plant. Rising sap stretched plant cells, causing the cells and hence the plants to grow; when a cell could stretch no longer, the plant withered and died. Tournefort thought this theory also explained the maturation and release of seeds. Observing the organs of plants — the ovaries of hellebore, aconite, and crown imperial, the fruits of spiny poppy, false dittany, toothwort, and the pods of the plant Caspar Bauhin named Lathyrus latifolius — Tournefort found that plants
released their seeds when the fibers in the ovaries dried and contracted, and he argued that when the ovaries opened, air entered and helped the seeds ripen.
Homberg, however, used Bourdelin's analyses to develop chemical explanations of how seeds and plants grew. Homberg concentrated on the maturation of seeds. Noting that unripe seeds yielded a lot of water, less oil, but more fixed salt than ripe ones, Homberg gave this account of their development:
… the organs of the young seeds contain only a watery and very fluid sap, which is not yet well digested; after these salty, earthy, and watery parts have mixed together more perfectly over time, they thicken and create this oil that forms little by little.
To strengthen his claim, Homberg compared young seeds with ripe fruits, nuts, and olives; stored in a dry place for three or four months they too yielded more, and thicker, oil than when they were freshly plucked. Homberg reasoned that the young seed resembled ripe fruit and became oilier as it matured. He also distilled fetid oil with quicklime; this diminished the oil, changed its color, and produced a lot of water. Homberg argued that the phlegm, salt, and earthy matter of young seeds "together create over time the quantity of oil that is found in ripe seeds." He believed he had separated the oily compound into the simple substances out of which nature had formed it. When rectifying oil with quicklime, for example, the quicklime separated fixed salt and earth from the oil; thus, Homberg reasoned, the oil must consist of salt, water, and earth.
All academicians agreed that only chemical analysis could explain the origin and nature of plant nutrients. Boyle, Helmont, and others stressed water and deprecated soil as the source of nourishment, and academicians investigated the role of rainwater and dew in the growth of plants. But savants who did not accept Helmont's view that water was the ultimate source of matter had to identify the origin of the nutriments found in water. Academicians debated whether these came from the atmosphere or from the earth, a question that turned on salts.
For Mariotte and Perrault, nutrients came intermediately from the earth but ultimately from the atmosphere when sulphur, saltpeter, and volatile salts fell in rainwater to the ground. Duclos disagreed with his colleagues. In his view, the fertile "fatty and sulfurous salts" formed in the soil, not in the air. As proof he cited his analysis of the waters that condensed inside and outside a concave vessel placed on the ground: dew had more volatile salt than either the rainwater or the air vapors that collected on the outside. His
arguments convinced Perrault, who conceded that the nutrients originated in the soil from living or decayed plants and animals or from whatever produced mineral salts in well water. Perrault thought such nutrients were cooked by the sun when they rose in vapors.
Applying chemical analysis to a concept of the food chain, Homberg developed a theory about the different origins of the different salts. He noticed that most plants were composed of three salts: fixed lixivial salt, volatile urinous salt, and volatile acid salt. Bourdelin's analyses showed that fixed lixivial salts and volatile urinous salts occurred only in the distillants of plants, of herbivorous animals, or of animals that ate herbivorous animals. The third substance, volatile acid salt, was found in soils, including ones that had no vegetation, and in plants that grew in all kinds of soil. Volatile acid salts thus came from the earth. The other two salts — fixed lixivial and urinous volatile — were therefore manufactured in the plant.
The Academy was divided on whether seeds and plants were active or passive. Did they simply absorb juices already prepared in the earth or did they transform what they took from the earth? Tournefort took the passive view. He believed that juices altered in the earth and became more or less suitable to various plants. Mariotte, Perrault, Duclos, and Homberg took the contemporary view that seeds and plants changed what they took from the soil, but they disagreed about how this happened. Mariotte believed the liquid was simply a vehicle for minerals and that plants used the mineral residue only after the liquid had evaporated. Perrault and Duclos believed the plant transformed both liquid and minerals, and Duclos thought it did so by coagulation. Homberg believed plants imbibed one kind of salt from the earth but created two additional salts themselves.