This article addresses René Descartes’ problematic interpretation of vegetative activities within his mechanistic programme for explaining all living operations. Initially, Descartes illustrates nutrition and digestion by means of the analogy between animals and hydraulic machines. His account has a glaring weakness: he fails to supply a functional explanation for vegetative operations. Several botanical notes collected in the Excerpta anatomica reveal Descartes’ later attempt to bridge this lacuna. His study of plants (1) provides him with material for an improved specification of vegetative activities, (2) helps to shape a mechanical vegetative power that regulates bodily constitution and growth, and (3) allows him to isolate a class of living beings. While a more thorough explanation of nutrition, digestion, and growth in mechanical terms surfaces, Descartes proposes plants as a suitable model organism to explain vegetative activities.