1918] THOM PSON—GNETALES AND ANGIOSPERMS 87 
the higher type. It is not my intention to discuss the origin of 
this scalariform vessel in detail. It may be pointed out, however, 
that such an end wall may have developed in one of two ways. 
(1) The scalariform perforations may be 
modifications of the scalariform bordered pits 
characteristic of the primary tracheids of all 
vascular plants and of the secondary tracheids 
of many ancient forms (Lepidodendron, Cala- 
mites, Bennettitales, etc). If this alternative is 
the correct one, we have in the angiospermic 
vessel of this type a retention of a very primi- 
tive form of pitting which has disappeared from 
the secondary tracheids of all plants above the 
cycads with the possible exception of such plants 
as Trochodendron, Tetracentron, etc. (6). With 
the exception of these perforations it is also 
absent from the vessels of all angiosperms, although JEFFREY 
and Core (3) regard as vestiges of vessels certain elements with 
this kind of pitting which they have found in wounded Drimys 
and which occur normally in Trochodendron and Tetracentron. 
According to one alternative, therefore, the angiospermic vessel has 
been produced when the scalariform bordered 
pits on the end wall of a tracheid lost their 
membranes and became perforations. At the 
same time the pits of the lateral walls were 
transformed into the familiar crowded circular 
ri- 
Fic. 
form vessel of birch. 
e. 
(2) On the other hand, the scalariform per- 
forations may have resulted from the fusion of 
Fic. ro.—Typical pits of the ordinary circular multiseriate type. 
e biospermic vessel Tn many angiospermic woods all gradations may 
from Vaccinium . d Iti 
corymbosum. be observed between scalariform and multi- 
seriate circular pitting. If this alternative 1s 
correct, the angiospermic vessel has not been derived from the 
Primitive tracheid with scalariform pits, but from the higher 
ordinary type of tracheids with circular bordered pits. The per- 
forations are therefore not retentions but new productions. It 
