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morphological grounds that Campbell (Mosses and Ferns, 1905, p. 
465) denies the homology of the vascular system of Equisetum 
with the central vascular strand of Sphenophyllum. He says of 
Equisetum, “the whole vascular system of the stem originates from 
the primary cortex or periblem, the original central tissue-cylinder 
giving rise only to the pith/’ The same arguments would apply 
in comparing the stems of Lygodium and Adiantum, or the very 
young and the mature individual of Dennstaedtia. That the vas- 
cular tissues of the individual fern are serially homologous follows 
from the observed facts of development. That they are phylogen- 
etically homologous in different individuals and different species is 
indicated by the exact parallelism, pointed out first by Jeffrey and 
Boodle, between the ontogenetic and phylogenetic series. Hence 
any negative argument based on the origin of tissues in primary 
meristem must fall. The developmental viewpoint is manifestly a 
guide to phyletic relationship. 
Shall we then be free to adopt the purely morphological basis, and 
consider similarity of structure equivalent to community of origin ? 
By no means. The single initial cell of the moss (Musci) sporo- 
phyte, for example, has no direct relation to that of Equisetum or 
the leptosporangiate ferns. For if we can think of any ancestor 
of the pteridophytes among the byrophytes, it must be in the 
Hepaticae and not the Musci. But no living hepatic shows any 
well marked apical growth in the sporophyte. Therefore it is most 
probable that growth by a single initial has arisen independently in 
mosses and ferns. Again, in true ferns the cortical and medullary 
tissues often show striking resemblances. But ontogeny teaches 
us that they have very different histories. They may be truly con- 
sidered as a unit, the fundamental tissue. But the parenchyma 
enclosed by the vascular ring is no more to be called cortex than is 
that outside to be called medulla. Hence a strict morphological 
basis of homology is not tenable. 
So we must come back finally to the conclusions which follow at 
once from morpho-physiological considerations. Those tissues are 
homologous whose form, function and position in the organism 
point to a common origin. And we may safely say that the normal 
primary vascular tissues of all vascular plants are truly homologous. 
