93 
The Leaf-strtcdure of Cordaites. 
In a section at right angles to the above, and nearly parallel 
to the bundles (fig. 7), the differentiation of the leaf comes out 
more clearly; the palisade cells are broad but regularly arranged 
in rows, the number of which differs according to the angle of the 
section, but averages roughly at four; the cells of the bottom row 
of palisade, separated from one another by slit-like lacunae (Fig. 7), 
abut, often in pairs, on the large cells of a row which in its turn rests 
on the elongated cells appearing between the palisade and the 
spongy parenchyma ; this last is composed of irregular cells chaining 
together and forming a highly lacunar tissue. The stomates were 
not sufficiently well preserved for description, but there are 
indications that they were frequent on the lower surface. 
In sections parallel to the leaf surface, which cut the bundles 
in median longitudinal section, the parenchyma runs in bands from 
bundle to bundle, separated by lacunae; these cells are the 
elongated ones described above, and are figured by Renault 1 , while 
tangential sections of the bundle cut the palisade or spongy 
parenchyma as the case may be. 
It is, however, in the bundles themselves that the chief 
interest lies. They are of the type usual in Cordaites , with well 
marked protoxylems and a wedge of centripetal xylem above 
(phot. 3 and 4, fig. 5, pi. IX.), the average diameter of the pro- 
toxylem being 10/x. and of the large centripetal xylem about 50/x. 
In longitudinal sections (see fig. 6) the protoxylem was seen to 
consist of well marked spiral elements, next to them are variously 
banded scalariform tracheides, and the largest one or two elements 
are closely pitted with five or six rows of bordered pits exactly 
similar to the elements of the secondary wood of the stem. The 
centrifugal xylem usually described is entirely absent from all 
bundles in these leaves. The deceptive cells in phot. 3 are clearly 
seen in the original section to be crushed and contents-bearing 
phloem-elements lying next the xylem-parenchyma somewhat 
obliquely, so that when the rest of the tissue is in focus they 
appear dark and thick-walled; but when they are separately 
focussed they are seen to be small thin-walled elements, quite 
similar to the rest of the phloem, next to which they are lying. 
That the absence of centrifugal xylem is a general character of 
this leaf is probable, as there seems no reason to suppose that 
these sections are specially near the tip or margin, in which case 
we might expect such a reduction in the bundle as occurs in recent 
1 Renault, “Sur quelques Tiges,” pi. 16, fig. 9. 
