2 1 2 Jeffrey . — On the Structure of the Leaf in Cretaceous Pines . 
transfusion tissues from the mesophyll. Still another noteworthy feature is 
the presence of strong hypodermal ribs beneath the epidermis. Fig. n 
shows another section of the same leaf still more highly magnified. All 
the features signalized above can now more clearly be made out. The 
transfusion tissue is particularly obvious in this figure. Fig. 14 shows the 
fibrovascular bundle somewhat highly magnified. To the left is a cavity, 
which marks the position of the phloem, now entirely gone. The dense 
although not very thick sheath, which surrounds the bundle proper, can be 
clearly distinguished in this figure. The most interesting feature of the 
figure, however, is the structure presented by the wood. It can be made 
out that there is a large amount of centripetal xylem present. It will be 
observed that the centripetal elements, in the centre of the bundle, begin 
about the middle of a radius drawn through the xylem and pass with 
a gradual enlargement of their lumen towards the dark sheath already 
described, upon which as a rule they immediately abut. Fig. 15 shows the 
central portion of Fig. 14, much more highly magnified. The seriation of 
the elements is particularly clear in this figure, as well as their relation to the 
peculiar bundle sheath. The centrifugal xylem is not broken into separate 
strands as is the centripetal wood. It does not come into direct relation 
with the thick-walled sheath above mentioned, except on its flanks. In 
other regions of the xylem the relation between the centrifugal elements 
and the transfusion tissues is brought about through the centripetal xylem. 
Fig. 12 shows a longitudinal section through the leaf-bundle somewhat 
to one side of the centre. The tissues do not appear as well preserved 
in this plane of section as in the transverse ; but it can nevertheless be made 
out that typical mesarch structure is present. The ringed and spiral proto- 
xylem elements occupy the centre of the figure, and on the left pass into 
reticulate elements, which are at once succeeded by pitted tracheids of the 
centrifugal wood. On the opposite side of the protoxylem are the thinner- 
walled and less well preserved elements of the centripetal wood, which are 
entirely pitted tracheids. On the extreme right is to be seen a portion of 
the very dense bundle sheath. This sheath is so darkly coloured in the 
fossil that it is almost impossible to resolve its structure even after the pro- 
longed action of such a bleaching agent as chlorine water. It is possible, 
however, to make out with sufficient pains that it is composed of very 
thick-walled transfusion elements, with small bordered pits. The elements 
of the dense transfusion sheath are very much longer and of much narrower 
lumen than are the more external transfusion cells. They in fact present 
a certain resemblance to tracheids. The broad zone of transfusion cells 
surrounding the dense sheath just described is made of cells of much 
thinner walls and only slightly elongated longitudinally. As has been 
mentioned at an earlier stage, these are all pitted with bordered pits, which 
are about twice as broad as those found in the cells of the transfusion 
