810 THE AMERICAN NATURALIST. (VoL. XXXIX. 
be found in their distribution and compounding as exposed in 
transverse section, but before considering these features, it may 
be well to discuss the few details relating to their aspects in 
longitudinal section. 
Thyloses are a general feature of the vessels in wood of the 
Salicacez, but they differ in some important essentials from 
those with which we are familiar in other woods, e. g., Catalpa 
or Pinus. They generally fill the cavity of the vessel so com- 
pletely (Figs. 6, 7) that in transverse section they seem to be 
entirely wanting. This is due to the fact that they undergo no 
longitudinal division, and as a result there is only one thylosis 
transversely, which is closely applied to the wall of the ves- 
sel. It will nevertheless be observed that in longitudinal sec- 
tion, instead of the modified spherical form usually assumed in 
woody stems, the thyloses of the Salicacez are really in the 
form of long, cylindrical cells several times longer than broad, 
but of very variable length. It is this last feature among others, 
which results in their apparent absence from the transverse sec- 
tion, since a given plane of section is most likely to pass be- 
tween terminal walls, and as the thyloses lie in a single series, 
there are no longitudinal walls to break up the cavity of the 
vessel. 
Apart from the well known structure which distinguishes all 
the vessels of the wood in angiosperms, it will be found that the 
radial walls are characterized by bordered pits which are chiefly 
somewhat distant, sometimes conspicuously so, and in conse- 
quence they are either oval or round (Fig. 10). It is rather 
the exception that they are so aggregated as to become hexag- 
onal. On the other hand, in the tangential section, the pits are 
invariably crowded together to such an extent that they are 
typically hexagonal (Fig. 11), and the space between contiguous 
ps ts represented by only a very narrow line of secondary wall 
jomed directly to the primary wall. These differences will be 
found m exact accord with the distribution of the pits on the 
radial and tangential walls as described for the transition zone, 
and they „Seem to imply that the transverse movement of the 
transpiration current must 
gential than in a radial d 
found to be the case in th 
