72 
Cor mack, — On a Cambial 
suiting from their division which is similar to that in an 
ordinary secondary thickening. 
Corroborative evidence is obtained by tracing the develop- 
ment of the vascular system. As is well known, each pseudo- 
whorl of segments derived from the tetrahedral cell of the 
punctum vegetationis produces a node with a leaf-sheath, and 
the internode beneath. 
At first there is no differentiation of the internode ; its de- 
velopment is the result of an intercalary growth at the base 
of the tissue produced from one of the pseudo-whorls. 
The course of the bundles is early marked out by the 
differentiation in the bud of the annular protoxylem of the 
stem, and of the leaf-bundles. Afterwards, during the 
elongation and general growth of the internodes, the proto- 
xylem-elements are separated and destroyed, giving rise to 
the carinal canals. Only protoxylem is lost in the formation 
of these canals. There is no appearance of destruction of other 
tissues ; and indeed the position of remains of protoxylem on 
the outer borders of the canals proves that the bundles have 
lost no other elements. Thus, in the mature internode, the 
only tissue missing from the bundle is the protoxylem. 
Now, on comparing a transverse section through an inter- 
node of the bud (Fig. %) with a transverse section through a 
mature internode (Fig. 5), it is seen that the number of cells 
in the radial thickness of the bundle is in both cases about 
equal. Hence it may be concluded that tangential cell- 
division is early arrested in the internodes. 
In the nodes on the contrary this is not so. Fig. 6 is from 
a transverse section through the node adjacent to the im- 
mature internode from which Fig. 7 was drawn. With this is 
to be contrasted Fig. 3, drawn from a corresponding section 
of a mature node. In the immature node there are fewer 
cells in the radial thickness of the bundle than in the mature 
node ; as yet the annular protoxylem is the only part of the 
wood fully developed, for the cell-division which produces the 
radial rows in which reticulate xylem is developed, has only 
recently begun. 
