John James, a distinguished botanist of the eighteenth century, who may be called the father of cryptogamic botany, was born at Darmstadt in 1687. He was educated as a physician in the university of Dillenburg, Giessen; but his attention was very early diverted from medical studies to the observation and discrimination of plants; nor does he appear to have ever followed any branch of the practice of physic. In botany he was strictly a practical observer, having addicted himself but little to the principles of classification, and not at all to the physiology of vegetables. Some branches of zoology occasionally engaged him, which an assiduous collector of plants, in their native situations, can hardly escape, so closely are these studies, especially that of insects and the lower tribes of animated beings, connected with botany. Dillenius, whilst at Giessen, wrote several papers for the Ephemerides Naturae Curiosorum, on American plants naturalized in Europe, on coffee, on opium obtained from poppies in Germany, with some minute critical remarks on Spargula pentandra, as well as on various cryptogamic plants. He published also a paper on leeches, and two species of papilio. He printed at Giessen in 1719 his Catalogus Plantarum sponte circa Giessen nascentium, a valuable little octavo volume, with figures drawn and engraved by his own hand, of the parts of fructification, particularly designed to illustrate the generic characters of plants, previously not well arranged or understood. In this work he established many new genera, which have for the most part kept their ground. His great merit as a general botanist consisted in a constant attention to the only sound principle of scientific botany, the discrimination of genera by the parts of the flower and fruit. This principle, first proposed by the great Conrad Gesner, Dillenius applied to practice, with a severer judgment and closer attention than perhaps any other person, from Gesner to Linnæus. The little book in question is arranged most inconveniently according to the times of the plants' flowering. Its preface, however, enters into the subject of classification, a subject to which young botanists are generally prone, but of which they as generally, after having embroiled it, take their leave, in proportion as they acquire more practical knowledge. Dillenius so far displayed his judgment, that he rather showed the faults of the systems of Tournefort, Knaut, and Rivièrus, than offered any thing of his own. This led him into some controversies, from which he soon disengaged himself, and never subsequently took up the question at all.

The great William Sherard, while returning in 1718 from Smyrna through Germany, met with Dillenius, whose scientific merit could not have escaped so eminent a botanist. He brought him to England in 1721, and excited him to publish, in 1724, that valuable enlarged edition of Ray's Synopsis of British plants, which has ever since been in general use, and which the editor enriched with engravings of his own. In this publication, compared with the Catalogus of the plants of Giessen above mentioned, we cannot but perceive the difference between an author working upon his own original materials, and the commentator or illustrator of the labours of another. Though Dillenius made numerous and correct additions to Ray's work, in the cryptogamic tribes at least, he rather confused than improved the other parts of the book, especially with regard to synonyms, in which department he was never supremely accurate.

In 1782 Dillenius published his magnificent Hortus Elthamensis, in two volumes folio, containing 324 plates, engraved on pewter, with his own hand. Their merit consists in their very great precision and fidelity. The descriptions, and historical as well as botanical remarks, render this a classical book in botany. Its style is good, and the whole performance is worthy of the author, and of his eminent patron, whose brother, Dr James Sherard, was the owner of the garden at Eltham, which furnished the Dillingen rich materials of this publication. Before this book appeared, its author was established at Oxford, in the new professorship founded there by the will of William Sherard, who died in August 1728, and who left L3000 for the purpose, besides his own library, manuscripts, and ample herbarium. Dillenius took the degree of M.D. in this university in 1735, though he had previously obtained the same rank at Giessen; and he now devoted himself to the completion of the Pinax, or universal collection of synonyms, which was Sherard's chief object in this foundation. The work was never finished; for indeed neither Dillenius nor any one else could even at that time be competent to it: still less, as botanists and botanical works multiplied excessively, was this undertaking practicable. The publications of Linnaeus soon rendered it unnecessary. That illustrious foreigner in 1736 visited Dillenius, who was desirous of fixing him here as his co-adjutor; but to this scheme there were several impediments. Nevertheless these distinguished men continued ever after in correspondence, certainly to the advantage of their common study, except in one but too important instance. We allude to the theory of the fructification of mosses, in which Linnaeus implicitly adopted the faulty opinion of the Oxford professor, contrary to his own better observation and judgment, taking the capsule for the anther. This leads us to mention the immortal work on which the fame of Dillenius rests, and which, in its way, will never be excelled, the Historia Muscorum, published in 1741, in one quarto volume, with eighty-five plates, drawn and engraved by the author. In this performance, laborious investigation, acute discrimination, supreme accuracy, and profound learning, are displayed beyond all example or comparison. Following inquirers, like the celebrated Hedwig, may, with better helps, have examined the same objects more deeply; but none has taken so complete a view of the subject, nor made so very few mistakes. No botanical book perhaps is so perfect in synonyms. Whether the labour of this undertaking was too much for the health of its author, or whether his sedentary mode of life was, on the whole, injurious, we have no particular information; but he began, soon after the publication of the Historia Muscorum, to complain of ill health and advancing age. He was of a short stature and corpulent habit, and died of an apoplexy on the 2d April 1747, in his sixtieth year. A picture of this distinguished botanist is preserved in the picture-gallery at Oxford, from which a print has been published in Sim's and König's Annals of Botany, vol. ii. Dillenius is said to have been amiable and respectable in his private character. He never married. His books, collection of mosses referring to his great work, with many drawings, especially of Fungi, were bought by his successor Dr Humphrey Sibthorp, and added to the Sherardian Museum, where they still remain.