How does an expert transmit expertise? What genres of scientific writing are available for doing so? Does the choice of genre matter in the long run? In this essay, I approach these questions by comparing two monographs published in the mid 1940s in the field of microbiology. While the works shared a concern with life at its smallest, they were written in different genres. One, entitled L’évolution physiologique: étude des pertes de fonctions chez les microorganismes, was a general survey of research on microbial nutrition. The other, called Pure Cultures of Algae: Their Preparation and Maintenance, was a manual of techniques for cultivating microscopic algae in test tubes.