564 Weiss . — The Vascular Branches of 
It will be seen that in the two rootlets in which the spirally- 
marked elements are described in the cortex they are found 
in the innermost layers of the cortex, and therefore within the 
very dark cells which are so often found, forming a more or 
less continuous ring in the outer cortex. It is hardly possible 
to say whether the darkness of the cell-walls in this layer, 
which was no doubt due to the cell-walls and not to the 
contents, marks them out as an impervious or protective 
sheath ; or whether the cell-walls, though differing in constitu- 
tion, and probably also in thickness, may not have been as 
readily permeable as those of the other cells of the cortex. 
Even if they were less permeable, or actually impervious, there 
may have been at intervals thin-walled elements similar to 
those of a thick-walled endodermis, and through such thin- 
walled cells the water taken in at the periphery might have 
reached the spiral tracheids on the inside of the cortex. 
In his memoir on Stigmaria , Williamson figures a trans- 
verse section of a rootlet 1 in which there appear, in close 
proximity to the protoxylem group, two other groups of 
spirally-marked tracheids in the parenchymatous sheath of 
the stele. These isolated groups of tracheids, together with 
the small tracheids immediately adjoining the metaxylem, 
Williamson considers to f constitute the monarch point, to 
which the remaining vessels of the bundle were added centri- 
petally.’ A similar condition had been previously figured by 
Williamson 2 in Part XI of his Organization of the Fossil 
Plants of the Coal Measure, and the same explanation given 
of its occurrence. Dr. Scott, in his manuscript notes, dissents 
from this interpretation because ‘ the elements at (/"') are too 
big and too remote from the protoxylem to form part of it.’ 
He considers that we have here another case of a tracheidal 
strand being given off from the protoxylem. This conclusion 
is undoubtedly correct, the difference in appearance between 
Williamson’s figure and that of Renault being due to the fact 
that the strand of tracheids runs very nearly parallel to the 
3 Williamson, W. C. (’87), PI. XI, Fig. 59 . 
2 Id. (*81), PI. LIII, Fig. 19 . 
