Different Types of Angiospermic Vessels. 189 
consideration differences in the rate at which the transformation of one type 
into another takes place. In one radial row it may take place more 
rapidly than in another. Also the first vascular elements of one row may be 
farther out than those of another. Therefore sections which are slightly 
oblique and include more than one row may give very deceptive appear¬ 
ances. When the transitions are studied carefully in a single row of cells it 
is found that the scalariform pitting appears first, is succeeded by the reticu¬ 
late type, and this in turn by the multiseriate circular type, just as in ferns 
and gymnosperms, and that in many forms the scalariform type persists 
through the secondary wood. 
Moreover, adequate comparisons of the secondary wood of specialized 
and primitive families shows that scalariform pitting when it occurs is 
generally in the latter (such as Piperaceae, Monimiaceae, Cupuliferae, 
Magnoliaceae, Bruniaceae). As in other cases, the different tendencies in 
the evolution of wood structures are expressed in different degrees in different 
cases. But in general highly specialized woods show multiseriate and not 
scalariform pitting on the side walls. 
Miss Bliss also argues that since in some cases in ferns the end walls 
are reticulate while the side walls are scalariform, therefore the end wall is 
more specialized than the side wall. Consequently in those angiosperms 
which show the scalariform condition on the end wall and multiseriate pits 
on the side wall, the scalariform must represent the more advanced con¬ 
dition. It seems to be stretching the doctrine of conservative regions rather 
far to say that one side of a cell must in all cases be more conservative than 
another side. Moreover, as a matter of fact there are plenty of reticulate side 
walls in ferns and many dicotyledons with scalariform side walls. The true 
explanation of the occurrence of multiseriate pits on the side walls and 
scalariform perforations on the end walls of the same segments appears to be 
that the scalariform pit is being discarded for the multiseriate, that since the 
side walls in dicotyledons are in contact with a great variety of smaller cells 
the scalariform pits loses its alinement and quickly becomes broken up, and 
that since the end wall is in contact with a similar end wall the scalariform 
condition tends to remain in this position. 
The view that the scalariform pit is primitive in angiosperms and not 
derived secondarily by the fusion of multiseriate circular pits is then 
supported by (1) the extreme improbability of such a remarkable example 
of reversed evolution, (2) the succession of vessels with the different types of 
pitting and transitions in the first-formed wood, and (3) the occurrence 
of the scalariform type in primitive families and the nearly universal 
occurrence of the multiseriate type in highly specialized families. 
If the scalariform perforation is primitive in angiosperms, being a 
retention and slight modification of the scalariform pitting of primitive 
gymnosperms and pteridophytes, there has been no fusion process in its 
