/ 
510 Bailey and Thompson.-—Additional Notes upon the 
xylem of Tetracentron , Trochodendron , Drimys , and other genera of the 
Ranales, are concerned, it is just as reasonable to interpret them as indicating 
that circular bordered pits are derived from scalariform bordered pits as 
vice versa. Indeed, if one considers that the laws of recapitulation, reversion, 
and retention cannot in certain cases be admitted and in others denied, the 
more abundantly developed scalariform pitting in young stems of D. Winteri 
and injured roots of D. colorata should be interpreted as indicating that 
Tetracentron , Trochodendron , and the various species of Drimys form 
a graded series, illustrating the replacement of scalariform by circular 
bordered pits. Therefore, in the absence of reliable evidence which might 
be considered to prove that the secondary tracheides of the Coniferae or 
similar forms have been or can be modified to form typical scalariform 
secondary tracheides, the possibility that the vessel-less Tetracentron , 
Trochodendron , and Drimys are descended directly from ancestors having 
tracheides with scalariform bordered pits deserves careful consideration in 
discussions concerning the origin of the Angiosperms. 
It is to be emphasized that, although the secondary xylem of Tetra¬ 
centron and Trochodendron is in general unlike that of the Coniferae or 
higher Gymnosperms, it resembles the wood of certain Palaeozoic and 
Mesozoic plants, e. g. Catamites , Protopitys figured by Solms-Laubach (10), 
and certain Cycadeoideae described by Wieland ( 13 ). In all of these 
plants, as well as in the primary xylem of many ferns, there are transitions 
between typical scalariform bordered pitting and 6 opposite \ 1 alternate ’, or 
uniseriate circular bordered pitting. Recently, one of the writers enjoyed 
the privilege of studying and photographing Dr. Wieland’s excellent slides 
of type material of Cycadeoidea Dartoni (Coulter and Chamberlain), Wiel. 
As is shown in PI. XVI, there is a striking similarity between the tracheary 
pitting of this fossil and that of Tetracentron and Trochodendron . The 
writers emphasize this similarity in tracheary structure, not as indicating 
necessarily close genetic relationship between the Ranales and Bennettitales, 
but in order to show that certain of the Pteridophyta and older Gymno- 
spermae possessed a more plastic and generalized type of pitting than that 
of the relatively stereotyped Coniferae. 
Conclusions. 
The scalariform tracheary elements that occur in injured roots of Drimys 
colorata are not vessel-like in structure. 
They are typical tracheides having transitional types of pitting, such as 
occur in many Pteridophyta, Gymnospermae, and Angiospermae. 
They occur in uninjured stems and roots of Tetracentron , Trochoden¬ 
dron , and Drimys , as well as in traumatic tissue. 
True vessels do not occur in the xylem of these genera. 
1 This genus is now placed in the Cycadofilices. 
