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Anatomy of the Leaf in the Osmundaceae. 
point of branching is approached from below, the end of the xylem strand 
of the trace on the side of the branch becomes somewhat thickened. The 
protoxylem strand on that side also elongates laterally and prolongs itself 
into the thickened extremity of the xylem strand so that it becomes 
immersed. There is now a certain amount of the metaxylem of the 
thickened end of the strand on the adaxial side of the protoxylem ; this 
adaxial xylem may be regarded as centripetal (Photos 3 and 5). The 
elongated protoxylem now divides into an inner and an outer lateral proto- 
xylem, the two becoming separated by a few elements of metaxylem 
(Photos 3 and 6). The inner protoxylem is still endarch, touching on the 
concave margin of the xylem strand, but the outer is mesarch, being 
completely immersed in the metaxylem of the thickened and laterally 
prolonged end of the same. The outer protoxylem, together with the 
metaxylem lying to the outside of it laterally, is now nipped off by the 
incursion of a groove or furrow of parenchyma beginning on the adaxial 
side and gradually passing through the thickened end of the xylem strand 
of the parent trace to the abaxial side (Photos 4 and 7). 
Sometimes this groove of parenchyma begins so soon that it reaches the 
immersed protoxylems before they have become separated from each other 
by metaxylem. The inner protoxylem is in this case for a while in contact 
with parenchyma both on the inside and the outside, and there is an isolated 
mass of centripetal metaxylem on its adaxial side (Photos 8 and 9). This 
soon joins up with the abaxial metaxylem on the outside of the inner 
protoxylem, which thus becomes normally endarch again. 
In some cases of branching the lateral protoxylem of the xylem strand 
passes completely into the thickened end and becomes definitely mesarch 
(Photo 10). In these cases it may divide as before, the outer protoxylem 
passing off with the branch-trace, the inner opening out into the concavity 
of the parent trace and becoming endarch once more. The inner protoxylem 
may, however, still remain mesarch for some distance after the departure 
of the branch-trace before becoming endarch (Photo 11). 
In some cases the mesarch protoxylem does not divide at all, but passes 
out as such with the trace. Here, of course, the xylem strand of the mother 
axis will have one protoxylem less above the branch than it has below 
(Photos 12 and 13). 
In the smaller branchings of this type, when the end of the xylem 
strand is but slightly thickened, the centripetal elements on the adaxial side 
of the dividing protoxylem are very few — three, two, or only one (Photo 14), 
It is obvious that these are transitional stages to the simplest type of 
branching already described. 
Another type of branching, which must be regarded as an advance on 
the type just described, is usually found in somewhat larger branchings. 
When the lateral protoxylem of the mother axis is prolonged laterally into 
