Shedding of Bothrodendron . 
225 
In a more recent paper in the Annals of Botany, July, 1914 , Mr. 
Watson (13) has discussed these two theories, objecting to the umbilical 
attachment theory on the following grounds : 
1 . The relative insignificance of the secondary thickening in any 
lepidodendroid plant as compared with that which would be required by 
M. Renier’s theory. 
2 . No ulodendroid scars are known in which the diameter of the 
umbilicus is more than a quarter of the diameter of the scar ; that is, the 
first stages as required by the 
umbilical attachment theory are 
unknown. 
3 . The weakness of the 
calamite branch analogy. 
4 . The evidence of struc¬ 
ture material shows no contrac¬ 
tion at the base. 
5 . In the two new sections 
described by Mr. Watson the 
whole base of branch is cut off 
by a thick layer of secondary 
tissue ; that is, there is a definite 
abscission layer. 
There are only two ad¬ 
ditional arguments which I 
should like to bring forward. 
For the sake of argument, 
Mr. Watson admits M. Renier’s 
contention that the leaf-trace 
markings on both halves of his 
specimen do not correspond in 
position. But if the figure of 
one half of the specimen is traced and the tracing reversed on to the figure 
of the other half, it will be seen that the leaf-trace markings agree very 
closely in position and are equal in number. 
The second point concerns the arrangement of the leaf-trace markings 
on the scar. In the specimens figured by M. Renier and in many other 
well-known figures such as those of Stur, the quincuncial arrangement 
of the leaf-traces on the trunk is continued on the lower part of the 
scar, and this was used as an argument that the scar, except the umbilicus, 
was of the same nature as the trunk—that it was, in fact, merely a flattened 
portion thereof. But this does not take into consideration the fact that on 
the upper part of the scar the leaf-traces are very differently arranged. On 
the abscission layer theory the whole area of the scar simply represents 
Q 
Text-fig. 2. Illustrates the abscission layer theory. 
The heavy broken line marks position of abscission 
layer; otherwise as in Text-fig. i. 
