Botanists in the Academy first had to show that the circulation of sap was plausible. Like Harvey, who reminded his readers of other circular motions in nature and claimed "as much right to call this movement of the blood circular as Aristotle had to say that the air and water emulate the circular motion of the heavenly bodies," Perrault compared the circulation of sap to the cycle of condensation and evaporation. Harvey and Perrault both dwelt on the differences between living creatures and other things, Harvey stressing the connection between heat and life, Perrault the "natural connections" that unite the various parts of the body. Mariotte emphasized another of Harvey's themes, the connection between movement and life. Harvey affirmed that motion is necessary to generate the heat that is associated with life and pointed out that blood clots when it does not move, and Mariotte observed that still liquids stagnate and become corrupt or death-like.
Pretheoretic analogy or plausibility rested here on lazy analogy. Plants and animals were living creatures that depended for life on the motion of liquids that transported vital heat to all their parts. These were seventeenth-century truisms about the nature of life. Better pretheoretic support came from the specific context in which circulation of the sap was proposed. Perrault and Mariotte believed that this was above all a question of nutrition.
In 1667 Perrault had introduced the theory of circulation as a way of explaining how plants are nourished. In the following year he cast his entire analysis in the context of whether plants and animals are nourished similarly. When Mariotte explained what aspects of blood he meant to compare with sap, he started with the reception of chyle by the lacteal veins in the
mesentery and their transmission of this food to the venous blood. Following this non-Harveian interpretation of the lacteal veins, Mariotte described the full circuit of the blood and then a hypothetical circuit of the sap:
Probably the ends of the roots imbibe liquid from the earth and carry it into the body of the root. From there it passes into small vessels in the stem; and then it is distributed to the branches and ends of the leaves. The remainder is carried along different small channels to the root to be perfected by a type of cohobation and in order to become a well-digested sap, appropriate for the nourishment of flowers and fruits.
When Mariotte published the theory, he called his treatise "On the Vegetation of Plants," and he embedded the circulatory theory in a broader discussion of the chemical composition of plants, their germination and growth, the origins of vegetable nutrients, and the effects of plants on other living creatures.
These were the general grounds on which circular motion of sap was plausible. In addition to the vague principle that motion is necessary to generate life-sustaining functions, academicians cited the more specific need for nourishment characteristic of all plants and animals. A functional rather than a formal resemblance between plants and animals stimulated the analogy. But to confirm it Mariotte and Perrault still had to show that sap actually circulated and to find structures in plants that resembled the circulatory organs of animals.