Conducting Tissue-System in Bryopjiyta. 29 
separated from the cortex than in the stouter species, while 
the cortex itself is distinctly thinner and the number of leaf- 
traces present at a given level decidedly less, only seven to 
ten being found outside the hydroid mantle (cf. Bastit, Fig. 30 
p. 269). There is but a single row of hydroids in the leaf- 
bundle between the leptoids and the lower (outer) row of 
starchy ‘ Leitparenchym,’ which last when it enters the cortex 
has, as Haberlandt states, much lighter walls than in P. com- 
mune. The inner row of ‘ Leitparenchym ’ loses itself in the 
cortex. The changes in form of the trace as it passes into 
the cylinder are quite similar to those in P. commtme. 
P. formosum has very much thinner-walled tissues than 
either of the two preceding species. Its central thick-walled 
hydrom-strand has a considerably smaller diameter, but its 
elements are very like those of P. commune. The peripheral 
hydrom-mantle is very thin-walled and relatively very broad, 
and the hydrom-sheath is well differentiated. The tissues 
outside this, however, including all but the surface-layer of 
the cortex, are quite thin-walled, and the leaf-traces are not 
at first sight easy to distinguish. Their behaviour seems, 
however, to agree in essentials with that obtaining in the 
other species. There are not more than ten present outside 
the hydrom in any given section. 
In P.piliferum the central cylindrical strand is large rela- 
tively to the diameter of the peripheral mantle, which is not 
very well differentiated from it. The hydroids are rather 
narrow. The hydroid-sheath is large-celled and thick-walled 
— very well-marked. The leptom-mantle is difficult to dis- 
tinguish in the transverse section. The leptoids are com- 
paratively few in number, their walls do not differ from those 
of the adjacent tissues, and their cells often contain granules 
which before treatment with iodine much resemble the starch 
of the other tissues. The leptoids of the mantle are seen, 
however, on longitudinal section to possess typical characters, 
and we have not found starch in them. On the other hand, 
the corresponding cells of the leaf-bundle and trace often 
do contain starch and show no differentiation from ordinary 
