184 
Thompson.—The Relationships of the 
The Widespread Occurrence of Transitions between 
SCALARIFORM AND SIMPLE PERFORATIONS. 
Transitions between scalariform and simple perforations (Fig. 1) are 
present in a great many species belonging to many different families. In a 
considerable number of species both scalariform and simple perforations 
may be found in the adult wood, and almost invariably these woods also 
show transitions between the two types. In many species simple perfora¬ 
tions only are found in the adult wood but scalariform ones are present next 
to the pith, and these species generally show beautiful stages in the loss of 
scalariform bars in the region just outside the scalariform vessels. In 
almost every family in which species are found characterized by simple 
perforations and others characterized 
by scalariform ones, species may 
also be found showing the inter¬ 
mediate stages. 
While on the whole the transi¬ 
tions are commoner in the lower 
families they can be found even 
among families placed very high sys¬ 
tematically. In the Caprifoliaceae, 
for example, beautiful cases are quite 
common. In Fig. 1 a group of transi¬ 
tional perforations are shown as 
seen in Symphoricarpos occidentalis 
(Hook.). For the details of the 
process the reader is referred to my earlier paper ( 5 ). Altogether I have 
noted such transitions in many species distributed among twenty-eight 
families from Casuarinaceae to Compositae. It may be concluded that 
at least in the great majority of angiosperms the uniperforate vessel has 
been evolved from the scalariform type by the loss of bars from the latter. 
and simple perforations in the vessels of Sym¬ 
phoricarpos occidentalis. 
Vessels with Reticulate Perforations. 
In some species there are perforations which are neither simple nor 
purely scalariform but which are in some degree reticulate. The reticu¬ 
lations are of several different sorts. The simplest kind is that which 
is not uncommonly associated with the scalariform type. Wherever 
scalariform pits or perforations are found, from ferns to angiosperms, there 
is a tendency for contiguous bars to be joined occasionally, either by a more 
or less definite cross-piece or as a result of their being not quite parallel. 
In some cases these connexions between contiguous bars become fairly 
frequent, and a more or less intricate net results. But the essential 
