PETERSON : DESCRIPTION OF NEW RODENTS 191 
On the other hand, the tissues of the central portion of the root, the stele, even 
though they may have become lignified or woody, are still permeable to these fluids, 
and thus can be readily entered by some of the bacteria of decomposition. 
A few sections were found showing more or less completely the entire structure 
of the root, but the detail of the vascular bundle could be made out only with con- 
siderable difficulty, as the cells were usually very dark and the structure mostly 
obliterated. Enough was evident, however, to plainly indicate that nearly all of the 
roots were those of angiosperms, the cells discerned being quite typical. Rather 
large trachese, with the customary rings and reticulations, together with longer cells 
of a smaller diameter, some of the latter also showing reticulations, were quite 
plainly to be seen in the stele. No pith cells were evident but the woody elements 
were enclosed by a well developed bundle sheath. Fragments of older roots with a 
strongly developed cortical region were found; in some of these the rectangular 
cortical cells were built up with all the regularity of brickwork, each successive layer 
being regularly and perpendicularly superimposed. 
One of the main structural differences between stems and roots lies in the man- 
ner in which branches originate. In stems the branches originate near the surface 
but in roots the branches originate on the vascular cylinder and burrow upward 
through the cortex thus disturbing the arrangement of the cortical cells. An ex- 
ample of this was found in one of the sections, as may be seen by consulting the 
figure. 
CARNEGIE MUSEUM, January 24, 1905. 
