68 
Cor mack. — On a Cambial 
Literary and Philosophical Society \ leaves little doubt that 
such forms constitute one group of closely related plants. He 
distinguishes three types : to these may be added a fourth, 
described by Dawson 1 2 , which completes a very closely- 
connected series. The relationship of these forms is shown 
in the following table : — 
Woody Wedges 
Interfascicular Tissue 
(a) Calamites 
Elements barred ; thin me- 
dullary rays. 
Cellular 
( b ) Calaniopitus 
Elements reticulated on 
radial walls; thin me- 
dullary rays. 
Cells elongated ; medul- 
lary rays. 
(c) Calamodendron 
(d) Eucalamodendron 
Elements barred ; medul- 
lary rays more massive 
than in {a) or (p). 
‘ True bordered pits or 
pseudo-scalariform slit- 
pored tissue.’ 
Prosenchymatous and 
partly selerenchyma- 
tous; medullary rays 
more massive. 
The distinguishing features of these four types are found in 
tissues formed in the course of secondary thickening. The 
barring of the elements is in all essential points identical 
with annular thickening. Williamson writes 3 — £ In several 
instances I have noticed that the vessels at the inner 
extremity of the wedge were barred, whilst those constituting 
its peripheral portion were reticulated.’ In fact the differ- 
ences between the four types are such as would be re- 
capitulated during the development of Eucalonnodendron, the 
most highly differentiated of the series. It is difficult to 
avoid correlating such differences with differences of bulk 
attained by the stems ; and on turning to Dawson we find 
‘ that under (a) and ( b ) there are some species in which the 
woody cylinder is very thin in comparison to the size of the 
1 Memoirs of the Manchester Literary and Philosophical Society, Third Series, 
vol. X, 1886-87. 
2 Geological History of Plants, London, 1888. 
3 Phil. Trans., 1871, p. 481. 
