
No. 464.] STUDE OF: TAR: SALICACEÆ 529 
xylem structure in all its parts, with each year of growth, and it 
is not unreasonable to consider that the same structural regions 
should be designated by the same terms wherever they may 
occur. In this sense, therefore, we should recognize the pri- 
mary growth, as well as the secondary growth of each subse- 
quent year, as composed in each case of protoxylem and 
secondary xylem. The protoxylem survives throughout the 
entire life of the plant. 
In the higher gymnosperms as also in the Dicotyledonous 
angiosperms, the case is quite different. In them the rings of 
secondary growth are in all cases united to and conterminous 
with the structure of the preceding year, and under these cir- 
cumstances, as well as for reasons which will again be referred 
to, the protoxylem experiences complete suppression and does 
not reappear after the growth of the first year. In this case, 
then, the primary growth consists of protoxylem and secondary 
xylem, while the secondary growth consists altogether of sec- 
ondary xylem, and in this we find one of the most important 
structural distinctions between the Cycadacez and all other 
types of gymnosperms, bringing the former into the closest 
relations with their ancestral forms and removing the other 
gymnosperms with the Cordaitales as the basal member of the 
series, to a distinctly higher phylogenetic position. 
In Cordaites, reduction has been developed to such an extent 
that the protoxylem no longer forms a constituent part of the 
annual rings of growth, since it is wholly confined to the growth 
ring of the first year where it constitutes a region of very con- 
siderable radial extent, gradually merging with the secondary 
xylem in such a way that there is no sharp line of demarcation 
between the two. Within this zone graduated transitions are a 
"well known and remarkable feature; but it is to be noted that 
in all these respects there is a further removal from the ances- 
tral forms and a distinct development along those lines of evo- 
lution which eventually issue in the characteristic structure of 
the higher Coniferales. As these latter are reached, it is to 
be observed that there is a constantly greater reduction in the 
ial extent of the transition zone, and the intermediate struc- 
tural variations which it presents, until, eventually, it becomes 
