76 
Cor mack. — On a Cambial 
Now the persistent wood of the Calamitae is almost wholly in 
radial rows, and is therefore regarded as secondary, the proto- 
xylem having been almost entirely lost in the formation of the 
canals. Further, the position of the cambium in Fig. n shows 
that the development of the wood is centrifugal. It follows 
that the inner portion of the wood, which is found only in the 
node, was developed before that which is common to both 
node and internode. 
The same considerations, reinforced by an examination of 
Williamson’s figures, makes it seem highly probable that 
secondary thickening began before the elongation of the 
internodes, proceeded simultaneously with that elongation, 
and continued after the internodes had attained their maxi- 
mum length. Now it was shown that the formation we have 
seen to result from cambial activity in the node of Equisetum 
originates in the bud, and that the side-groups of xylem in 
the internode are formed later during the elongation of the 
internode. If, then, our interpretation of the development of 
the wood in Calamitae is correct, the two cases are similar. 
But whether or not it be accepted that secondary thickening 
of Calamitae began in the bud, this much is certain, that in 
its main features the course of development of wood of Cala- 
mitae was similar to that of Equisetum ; the differentiation of 
the later wood began in the nodes and afterwards extended to 
the internodes. This conclusion is not drawn from hypothe- 
tical considerations, but is the logical inference from the results 
of observations made by Williamson unbiassed by any theory. 
If we imagine the cambial development of Equisetum ex- 
tended through the internodes, and also become interfascicular, 
we produce a picture of the state of affairs in Calamitae. 
In the Equisetaceae, in correlation with their more or less 
aquatic habit, the xylem is greatly reduced ; its mechanical 
function is performed in part by the collenchyma, and its 
duties as a water-carrier and storer are undertaken in great 
measure by the carinal canals, assisted sometimes by the 
central canals 1 . The carinal canals, however, do not extend 
1 Strasburger, loc. cit. p. 438. 
