Historia Plantarum was written some time between c. 350 BC and c. 287 BC in ten volumes, of which nine survive. In the book, Theophrastus described plants by their uses, and attempted a biological classification based on how plants reproduced, a first in the history of botany. He continually revised the manuscript, and it remained in an unfinished state on his death. The condensed style of the text, with its many lists of examples, indicate that Theophrastus used the manuscript as the working notes for lectures to his students, rather than intending it to be read as a book.
Historia Plantarum was first translated into Latin by Theodore Gaza; the translation was published in 1483. Johannes Bodaeus published a frequently cited folio edition in Amsterdam in 1644, complete with commentaries and woodcut illustrations. The first English translation was made by Sir Arthur Hort and published in 1916.
The Enquiry into Plants is in Hort's parallel text a book of some 400 pages of original Greek, consisting of about 100,000 words. It was originally organised into ten books, of which nine survive, though it is possible the surviving text represents all the material, rearranged into nine books rather than the original ten. Along with his other surviving botanical work, On the Causes of Plants, Enquiry into Plants was an important influence on science in the middle ages. On the strength of these books, the first scientific inquiries into plants and one of the first systems of plant classification, Linnaeus called Theophrastus "the father of botany".
Theophrastus's two plant books have similar titles to two books on animals by his mentor Aristotle; Roger French concludes that he was effectively "doing a peripatetic exercise" in identifying regularities in and differences between plants, in the manner of Aristotle with animals. However, he went beyond Aristotle in describing seeds as parts of the plant; Aristotle, French argues, would never have described semen or embryos as parts of an animal.
Theophrastus made use of a variety of sources for the book, including Diocles on drugs and medicinal plants. Theophrastus claims to have gathered information from drug-sellers (pharmacopolai) and root-cutters (rhizotomoi). Plants described include poppy (mēkōn), hemlock, (kōnion), wild lettuce (thridakinē), and mandrake (mandragoras).