Different Types of Arigiospermic Vessels . 191 
pits. These latter have also on the side walls become broken up to form 
multiseriate circular pits through a reticulate condition. The reticulate end- 
wall perforations are associated with this, or in some cases may represent 
special anomalies. 
In Gnetales, on the other hand, the uniperforate vessel has undoubtedly 
been produced by the fusion of smaller perforations derived from circular 
bordered pits ( 5 ). There is no scalariform or reticulate condition in the 
secondary wood. Occasionally, since the pits are haphazardly arranged, 
two perforations derived from bordered pits may, in specially selected 
places, stand on the same horizontal level. But to use such specially chosen 
portions of an end wall as examples of the scalariform condition, as Mac- 
Duffie ( 3 ) has done, is to use the term scalariform in a sense quite different 
from that which it has always conveyed. 
Summary. 
v 
1. Transitions between scalariform and simple perforations are found 
in the vessels of numerous species of angiosperms belonging to many 
different families from the lowest to the highest. 
2. In a few species reticulate perforations accompany the other types. 
In some cases they are like the reticulate pitting which frequently accom¬ 
panies scalariform pits; in other cases a regular net with isodiametric 
individual openings is found. In the former the individual openings may 
fuse irregularly and produce simple perforations; in the latter the net 
appears to be lost as a whole. 
3. The evidence, which is reviewed, indicates that the scalariform 
pitting of angiosperms is a retention of the primitive scalariform pitting of 
the older gymnosperms and pteridophytes, and is not produced secondarily 
by the fusion of circular multiseriate pits. On the other hand, it has given 
rise to the circular pits of the side walls through a reticulate condition. 
It has also given rise to the scalariform perforations of the end walls. 
4. Since the scalariform perforations are not the result of a fusion, 
the reticulate perforations are not the result of a variation in this fusion. 
They represent rather a special or abnormal condition generally found 
in specialized forms. 
5. The perforations of Gnetales are not genetically related to any of 
the types found in angiosperms. 
University of Saskatchewan. 
O 
