28 Tansley and Chick. — Notes on the 
hydroid strands in the stelar mantle, the elements actually 
belonging to the different traces can no longer be dis- 
tinguished. We have in fact a stele entirely composed (with 
the above-noted exceptions) of leaf-traces, a state of things 
which corresponds to that obtaining when all the bundles in 
the stem of a Phanerogam are ‘ common bundles.’ The 
implications of this will be discussed in the third section of 
these ‘Notes.’ Coesfeld (op. cit., pp. 156-8) objects to the 
separation of the central thick-walled strand from the outer 
thin-walled mantle. But the sharp line of separation between 
the two is obvious enough as an anatomical fact in P. commune 
and often in P . juniper inum, and in all the species a difference 
can be made out between the peripheral and the central 
tissue. It is perfectly true that ‘the whole tissue of the 
central strand is homogeneously laid down behind the grow- 
ing point,’ but that is only what one always finds when two 
tissues of similar form are developed in contact. ‘ Develop- 
mental ’ facts of that kind can have no bearing on morpho- 
logical conclusions. Variability in thickening of the walls 
of the peripheral zone, both in different species and also in 
different stems belonging to the same species, certainly exists, 
and sometimes the mantle shades gradually into the central 
strand, but this assimilation of the neighbouring cells of 
regions primitively distinct is again a familiar enough fact 
throughout plant-histology, and cannot we think be set against 
the kind of general evidence we have adduced in the third 
section of these Notes. 
The aerial stems of P. juniper inuni. for mo sum and piliferum, 
like their rhizomes, resemble that of P. commune in all funda- 
mental respects, though their tissues are rather less well- 
differentiated, on the whole thinner-walled, and consequently 
more difficult to separate. The diameter of the hydroids in 
the central thick-walled cylinder of P. juniperinum is less 
than in P. commune . The peripheral hydroid mantle is 
thicker-walled, the distinction between the two being less 
sharp. The other layers of the stele are well marked, the 
pericycle being distinctly thinner-walled and more sharply 
