A ngiosperms Tetracentron, Trochodendron, and Drimys. 509 
secondary xylem of uninjured stems and roots of Drimys . Furthermore, 
the occurrence and distribution of these structures appears to be influenced, 
to some extent at least, by purely physiological and ecological factors. 
Since the occurrence oftracheides with scalariform bordered pits cannot be 
explained satisfactorily upon the assumption that Tetracentron, Trochoden¬ 
dron > and Drimys are ‘ evascularized ’ Dicotyledons—‘ degenerate ’ forms 
whose ancestors possessed true vessels in their secondary xylem—what is 
the probable significance of these interesting tracheary structures? 
From the morphological and phylogenetic points of view, it may be 
assumed that (1) Tetracentron , Trochodendron , and Drimys are descended 
directly from ancestors which possessed scalariform tracheides in their 
secondary xylem, or (2) they are descended from ancestors having tracheides 
with circular bordered pits. If the latter hypothesis be accepted, the 
scalariform tracheides in the secondary xylem of Tetracentron, Trochoden- 
dron, and Drimys must have been acquired de novo, e.g. by the horizontal 
elongation of single circular bordered pits, by the fusion of rows of circular 
pits, or by a transfer of the impulse to form scalariform pits from the 
primary to the secondary meristem— homoeosis , Leavitt ( 7 ), 
In this connexion it is desirable to discuss in greater detail the types 
of tracheary pitting that occur in Tetracentron, Trochodendron, and Drimys . 
The secondary xylem in the first two genera is characterized by possessing 
clearly differentiated growth layers or annual rings (PL XVI, Fig. T). The 
tracheides in the first-formed portion of the concentric layers are com¬ 
paratively large and thin-walled and have numerous scalariform bordered 
pits in the radial facets (Text-fig. 1, and PL XVI, Fig. 6); those in the last- 
formed portion of the rings are small, thick-walled, and fibre-like, and have 
scattered circular bordered pits (Text-fig. 5, and PL XVI, Fig. 6). Between 
the typical scalariform and fibre-like elements, intermediate types of 
tracheides with transitional types of pitting occur. These cells show all 
gradations between scalariform and circular bordered pits. In certain cases 
the scalariform bordered pits are replaced by two or more smaller pits 
(Text-fig. 2) ; in others, by single oval or circular pits (PL XVI, Fig. 6). 
When the former phenomenon occurs, the circular bordered pits form hori¬ 
zontal and vertical rows, so-called ‘ opposite ’ pitting. This type of pitting 
may grade into the type which occurs in the fibre-like tracheides, by a simple 
process of reducing the number of vertical rows to one, and by a gradual 
decrease in the number of pits in the remaining row. Occasionally, the 
vertical rows become more or less ‘ staggered ’, producing the so-called 
‘alternating’ type of pitting. In the genus Drimys the secondary xylem 
may be composed of similar combinations of tracheary elements or of one or 
more of the intermediate or transitional types (PL XVI, Figs. 7, 8, 9, 10, 
and 12). 
In so far as the transitional types of pitting, which occur in the secondary 
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