Biosemiotics is the field of studying sign processes in and among living organisms in nature. If Jakob von Uexküll started to explore the semiotic processes in nature, it was Peirce's theory of signs that provided the analytical tools to the field via Thomas Sebeok. This article examines the way Peirce's theory of signs was adopted in biosemiotics, focusing on the ways in which the problem of the mental interpretant has been solved in biosemiotics literature. Although this problem was relatively benign when the triadic sign model was applied to animal communication (zoosemiotics), it becomes more problematic in the case of plants (phytosemiotics). By focusing on Peirce's analogy of the sunflower, I suggest that Peirce was well aware of the problem of the mental interpretant when he took that example. While biosemioticians circumvented the problem by referring to Peirce's idea of habit and the category of thirdness, I argue that the role of indexicality has played a more important role in establishing that a potential interpretant is sufficient for starting a flow of meaning. The question of the actual mental interpretant is also central in the development of Peirce's semeiotic theory, and further metaphysical implications for biosemiotics have to be clarified in the future.