552 Sinnott and Bailey . — The Origin and 
are to be regarded as plants which have suffered reduction from a woody 
condition, and not as the most ancient type of Angiosperms. The whole 
problem centres about the question as to whether cambial activity has 
arisen in a continuous or a discontinuous fashion, and it will therefore be 
worth our while to consider what evidence is presented by a study of the 
mode of origin of secondary growth in other groups of plants. 
In the ancient Lycopodiales from the Palaeozoic we have probably to 
deal with forms in which cambial activity had but recently made its 
appearance, for in most species the ring of secondary wood is narrow in 
proportion to the primary stele, and in many instances it seems not yet to 
have appeared. Whenever present, it is always perfectly uniform and 
continuous at its origin. Unlike that of the Lepidodendrids, the stem of 
the ancient Calamites was provided with a ring of separate primary bundles, 
and in many cases the cambium laid down xylem opposite these and 
parenchyma opposite the gaps ; but even here there is no evidence that it 
was not continuous from the first, or that all parts of it were not simulta- 
neous in appearance. Lyginodendron and Cordaites also possessed distinct 
and separate primary bundles, but the secondary tissue was in both 
instances perfectly continuous, as in the previous cases. In some very 
young stems of Lyginodendron , in fact, the very first formed secondary 
wood has been preserved, and it is evident that the entire ring appeared 
simultaneously and was not influenced at all by the position of the primary 
bundles. 
In all these groups the primary wood was centripetal in its forma- 
tion, and was not in intimate contact with the zone of secondary wood. 
In the Conifers and Angiosperms, however, centripetal wood is either very 
much reduced in amount or has disappeared, and the protoxylem has 
become closely attached to the centrifugal primary xylem, which in turn 
is intimately associated with, and merges gradually into, the secondary 
wood. It is a noteworthy fact that in such forms the cambium does not 
arise in the same continuous and uniform line as in the lower groups, but 
that its appearance in the young twig is much more irregular. This 
is evident to some extent in the Conifers, but even more in the woody 
Angiosperms. It is most strikingly apparent, however, in those herbaceous 
forms where secondary wood is produced long before secondary radial or 
parenchymatous tissues appear. In view of the evidence from the lower 
forms which we have discussed, it seems more reasonable to consider this 
irregularity of the cambium as a recent rather than as a primitive 
phenomenon, and to regard it as due principally to the close attachment 
of the primary to the secondary wood and the gradual modification 
of cambial activity, originally uniform, in conformity to the position 
of the primary strands. 
That a uniform and continuous cambium is a primitive condition is 
