Solving the Problem of Crucial Dissimilarities
Failure to find equivalent organs weakened the analogy seriously. The absence of what Hesse calls horizontal analogues meant that the causal mechanism was missing in plants. Academicians were reluctant to discard the circulatory hypothesis altogether, once they had found evidence that sap descends. But purported equivalents, putative valves, and two-way vessels were inadequate to explain the principal problem of the circulatory analogy, the very issue Perrault had identified with his opening words in 1668: how does sap rise in the first place?
One escape from the failed analogy was to compare plants with "lower" animals, whose anatomical organization was less differentiated. Such a move was consistent with the Harveian model, for Harvey had noted that valves are "not present in all animals" and are not "made with equal skill in all the animals in which they are present." Harvey had also allowed for circulation in animals that lacked hearts, and plants seemed similar to what Harvey had called "plant-animals." These were creatures such as oysters
and earthworms, which had only rudimentary hearts or no hearts at all, because they were too small, too cold, too soft, and "too little differentiated in their structure." Harvey admitted that they did not need "a propulsive organ to transmit food to their extremities." Their bodies were limbless and homogeneous. In them, ingestion "and expulsion of food is an in and out movement produced by contraction and relaxation of the body as a whole." They need no heart because "they use the whole of the body as such and an animal of this sort is in effect nothing but a heart."[24]
Valveless veins and heartless creatures, therefore, offered two escape routes to scientists who used Harvey's theory as a model for plants. A heart, valved veins, and pulsating arteries were not necessary for circulation in all animals, since the entire organism might serve as a propulsive mechanism, driving nutrients and excrement through itself. But plants differed from the plant-animals: they tended to be larger, had a more differentiated structure, and were stationary. Botanists who took the view that the entire plant could propel the sap upward would have to look for an external motive force. To show how sap rose at all, therefore, academicians had to look beyond anatomical or structural identities. They had to seek nonbiological forces operating outside or within the plant. Botanists still sought a pump, not organic but figurative, that could impel sap in a direction contrary to its natural downward flow.
In the absence of a biological causal mechanism, academicians turned to two explanatory modes: chemical and physical. The former operated inside the plant and was part of the normal physiology of a living creature. The latter could be either external or internal, depending on what kind of phenomenon was cited. Chemical explanation was consistent with the Harveian model, for the digestion of nutrients was thought to be a chemical process that resulted in effervescence and rarefaction of the digested substances. The physical explanation, on the contrary, relied on concepts of air pressure and capillary action unknown to Harvey. Both chemical and physical mechanisms were invoked by Perrault and Mariotte, who thereby diluted the biological model with nonbiological explanatory mechanisms. But the physical model itself was uncertain, since scientists in the late seventeenth century were unclear about the causes of capillary action. Thus the analogy with the circulation of blood, which had a visible and organic causal mechanism, led botanists to adopt as a causal mechanism a mysterious phenomenon that had only recently been investigated. When the analogy between the motions of blood and sap could not be sustained, academicians related the rise of sap to capillary action and air pressure.
When those explanations seemed inadequate they cited the chemical phenomena of effervescence and rarefaction.