CHAP, ii.] Phytotomy in the Eighteenth Century. 253 



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 phrase- 

 ology 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 Wolffs 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 l 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 Mus- 

 corum ' (1782) and afterwards in his 'Theoria Generationis' 



1 Johannes Hedwig, the founder of the scientific knowledge of the Mosses, 

 was born at Kronstadt in Siebenbiirgen in 1730. Having completed his 

 studies at Leipsic, he returned to his native town, but was not permitted to 

 practice there as a physician because he had not taken a degree in Austria. 

 He consequently went back to Saxony and settled first at Chemnitz, and in 

 1781 in Leipsic. Here he was appointed in 1784 to the Military Hospital, 

 and became Professor extraordinary of Medicine in 1786 and ordinary Pro- 

 fessor of Botany in 1 789. He died 1 799. He commenced his botanical studies 

 as a student at the University, and continued them in Chemnitz under trying 

 circumstances, till as Professor he was free to devote himself entirely to them. 



