This is in all essential points Wolff’s theory. We may omit his statements on the subject of the first formation of leaves at the growing point and of the development of the parts of the flower, as well as his physiological views on food and sexuality, which remained for a long time without influence on the growth of opinion, and mention only his doctrine of the growth of thickness of the stem. The stem is originally the prolongation of all the leaf-stalks united together. As many bundles of vessels are formed in the developed stem as there are leaves springing from the vegetative axis; each leaf has a single vascular bundle belonging to it in the stem, in modern phraseology an inner leaf-trace. The union of these bundles from the different leaves forms the rind of the stem; but if the leaves are very numerous, their descending bundles form a closed cylinder, and if the stem is perennial, the fresh production of leaves every year produces new zones of wood of this kind every year, which are the yearly rings. This view of Wolff’s on the growth of the stem in thickness bears an unmistakable resemblance to the theory afterwards suggested by Du Petit-Thouars, according to which the roots which descend from the buds are supposed to effect the thickening of the stem. The contests between Mirbel and his German antagonists at the beginning of the present century will bring us back again to the more important points in Wolff’s theory of the cell. Contemporary botanists paid less attention to the ‘Theoria Generationis’ than they did to Hedwig’s[71] phytotomic views, not on the formation of cells, but on the structure of mature tissue. HEDWIG had given various figures and descriptions of phytotomic subjects in his ‘Fundamentum Historiae Muscorum’ (1782) and afterwards in his ‘Theoria Generationis’ (1784); but he treats these topics at greater length in his treatise ‘De fibrae vegetabilis et animalis ortu,’ published in 1789, and known to the author of this work only imperfectly from quotations in later writers. Hedwig’s figures of histological objects appear to be better than those of any of his predecessors; they show that he used strong magnifying powers, and that his glass had a clear field of sight. His defect lay in preconceived opinions and hasty interpretation of what he observed. In order to refute Gleichen’s view of the stomata in ferns, he demonstrated the existence of these organs in many phanerogams, and observed the opening of the slits, which he named ‘spiracula.’ On the epidermis which he had removed for the purpose of these observations he saw plainly the double contour lines bounding the epidermis-cells, and therefore the cell-walls, which are at right angles to the surface. These he took for a particular form of vessel, and called them ‘vasa reducentia’ or ‘lymphatica,’ and afterwards ‘vasa exhalantia,’ and he thought that he had found them again in the interior of parenchymatous tissue,