28 BIOLOGICAL SOCIETY OF WASHINGTON. 



The so-called natural system of classification in botany is 

 based primarily upon the reproductive function. As founded 

 by Jussieu it was exclusively .so based, but DeCandolle under- 

 took to introduce a new principle, viz., that of the structure of 

 the axis or trunk, by which he separated exogenous from 

 endogenous plants, and supposed that this line could be drawn 

 betw T een monocotyledons and dicotyledons, erroneously includ- 

 ing the gymnosperms in the latter of these groups. The 

 weight of his authority not only long retarded the discovery of 

 the true position of the gymnosperms as the immediate de- 

 scendants of the crytogams, but it had the further effect of 

 barring out the important truth which vegetable paleontology 

 has at last made clear that there once existed a large class of 

 exogenous cryptogams. 



Origin of Exogeny. — It has long been known that the Stig- 

 marias of the coal measures possessed an exogenous structure, 

 and as early as 1839 Brongniart discovered that the stems of 

 Sigillaria elegans consisted of a medullary center surrounded 

 by a thin exogenous zone within a thick cortex. The woody 

 zone was shown to be composed of distinct wedges separated 

 by medullary rays. It is now known that nearly or quite all 

 coal plants having the external characters of Sigillaria have 

 this exogenous zone. It was also early discovered that certain 

 coal plants with the general appearance of Calamites exhibit 

 an exogenous structure, and it was at first supposed that these 

 must be something very different, and they were accordingly 

 called Calamodoidrou . Professor Williamson has shown that 

 all true calamites have an exogenous structure of a very defi- 

 nite character. He has also proved that the distinction drawn 

 between Sigillaria and Lepidodendron based on this character 

 is not tenable, since some true L,epidodendra also show the 

 woody zone and medullarv ravs. 



