124 AMERICAN JOURNAL OF BOTANY [Vol. 9. 
appeared in those roots whose surrounding medium afforded a scanty air 
supply. Norris reported that the cortex of roots growing in water is of a 
very flimsy and indefinite nature and is apt to fall to pieces when an at- 
tempt is made to section the roots. For roots grown in sawdust he re- 
FiG. 2. Cross section of root of Salix nigra, showing four large air spaces and a four- 
rayed condition of the cortex. X 90. 
ports that air spaces occur throughout and appear to be formed by the 
breaking down of groups of cells, the cell walls being in many cases not 
completely broken and stretching across the air spaces. 
The conditions found in Salix nigra correspond to those described by 
Norris for Zea Mays. The cortical 
root tissue of Salix appears very loose, 
and, as has been stated, the airspaces 
are well developed. Remnants of 
cell walls stretching out into these 
air spaces lead the writer to conclude 
that this provision for aeration results 
from the breaking down of cortical 
cells. 
When roots were removed from the 
cuttings to be placed in killing fluid, 
these air spaces appeared as continu- 
ous air passages parallel with the long 
axes of the roots. 
The accompanying figures will 
^ ^ . f . f o 7- serve to illustrate the kind of ma- 
FiG. 3. Cross section of root of Sakx , • , • • , • c 
nigra, showing five large air spaces and a Serial used m determmmg the Size of 
five-rayed condition of the cortex. X 90. cells. 
