493 
Origin of the U lodendroid Scar . 
Finally, in the specimen under discussion, another cambium arising 
in the deeper layers of the outer cortex has developed a considerable 
amount of secondary tissue within the main periderm. This cambium 
appears to have spread to the middle cortex of the base of the branch, 
where a thick layer of secondary tissue, indistinguishable from that of the 
main stem, has been formed. In fact, the final result is that the separative 
tissue across the base of the branch has completely assimilated itself to the 
secondary cortex of the main stem. This fact explains some of the ap- 
pearances by M. Renier in impression material of Ulodendron. 
That the tissue I have just described is really an abscission layer is, 
I think, certain from its close resemblance to the ordinary mechanism of 
leaf-fail of dicotyledonous trees, and to the cladopsis of Agathis . 
It thus seems to be almost certain that the typical ulodendroid 
scar is a thickened abscission layer at the base of a laterally issuing 
branch. 
General Discussion of Ulodendron. 
It will be of interest to examine Ulodendron , and also Halonia , in the 
light of this new knowledge. 
As Dr. Kidston pointed out many years ago, three types of the 
ulodendroid condition occur : 1 
1. With the scars oval, well separated, and with a central umbilicus. 
2. With the scars oval, well separated, and with an eccentric 
umbilicus. 
3. With the scars circular, close together, and with a central 
umbilicus. 
Specimens at Manchester suggest that there is still a fourth type. 
4. With the scars circular, well separated, and with a central 
umbilicus. 
It is useful to consider the meaning of the difference between these 
types. The difference in the shape of the scar is easily seen to be 
entirely dependent on the direction of the branch ; circular scars can only 
be produced when the branch issues horizontally and at right angles to the 
surface of the trunk. 
Oval scars will be produced when the branch does not leave the surface 
at right angles, but inclines upward. Its section by the outer surface will 
then be nearly an ellipse. 
The position of the umbilicus is of more interest ; it is certain that 
in those cases where the scars are oval and the umbilicus eccentric, the 
surface of the scar was, during life, a deep conical pit. Casts of this pit are 
very common, and that it was present is always obvious even from the 
examination of crushed specimens. 
1 Loc. cit. 
L 1 2 
