Chap.v Paleobotany, 1860-igoo 145 



whose publication ranged over the years 1872-93. It was 

 in connexion with his examination of this group that he 

 took up the position already attributed to him in opposition 

 to the French school, that secondary wood was developed 

 in the Vascular Cryptogams. Brongniart had studied 

 Lepidodendron Harcourtii and the allied genus Sigillaria, 

 as long ago as 1839, and had separated them widely from 

 each other on the ground that the last named, showing 

 secondary thickening, must be phanerogamic. Williamson 

 showed that L. Harcourtii was exceptional in this particular 

 and that most Lepidodendra, like the Sigillarias, increased 

 in thickness through cambial activity. Williamson showed 

 further that the two genera agreed in anatomical structure 

 and must be regarded as members of the same group. 



The structure of L. selaginoides was investigated later 

 by Hovelacque in 1892. 



The ligule of Lepidodendron was first identified by Stur 

 in 1892 ; the discovery was confirmed by Solms-Laubach 

 and by Hovelacque in the same year. The complete 

 anatomy of the leaf was worked out by Renault in 1896. 



The anatomy of the non-ribbed Sigillarias, first described 

 by Brongniart in 1839, was reinvestigated by Renault 

 and Grand'Eury in 1875. Bertrand described the ribbed 

 form 5. elongata in 1899. 



The curious fossils known as Siigmaria, which had been 

 found to occur in conjunction with Sigillaria and were 

 commonly held to be the roots of that genus, were care- 

 fully studied by Williamson and were ascertained to occur 

 also on stems of Lepidodendron. Their exact nature re- 

 mained undetermined at the end of the century. 



The character of the fructification of Lepidodendron was 

 thought to have been ascertained in 1880, when D'Arcy 

 Thompson showed that the fossils known as Ulodendron, 

 thick shoots bearing cones attached to them laterally, were 

 the flowering branches of the trees. Some doubt, however, 



GREEN K 



