As a professor of botany at the University of Jena, he wrote Contributions to our Knowledge of Phytogenesis (1838), in which he stated that all plants are composed of cells.

Thus, Schleiden and Schwann became the first to formulate what was then an informal belief as a principle of biology equal in importance to the atomic theory of chemistry.

Although Schleiden was not Jewish nor a historian by profession, he was noted for his defense of Judaism and against antisemitism, and wrote two works, Die Bedeutung der Juden für die Erhaltung und Wiederbelebung der Wissenschaften im Mittelalter (1877) and Die Romantik des Martyriums bei den Juden im Mittelalter (1878), published in English as The Sciences among the Jews Before and During the Middle Ages and The Importance of the Jews for the Preservation and Revival of Learning during the Middle Ages.

In a lecture on the "History of the Vegetable World" published in his book Die Pflanze und ihr Leben ("The Plant: A Biography") (1848) was a passage that embraced the transmutation of species.

[3] American composer Harriet P. Sawyer set one of his poems to music with her song “Die ersten Tropfen fallen.”[12]