1917] BRUSH—SYCAMORE WOODS 483 
Minute structure 
The pith rays are the only reliable means for identifying the 
woods of the sycamores. As viewed in the tangential section, the 
pith rays are broadest horizontally in the common sycamore and 
narrowest in the California species; the rays are lowest vertically 
in the common sycamore and highest in the California species. 
The rays of the Arizona sycamore are intermediate in character. 
These characters can readily be seen under the microscope. 
VESSELS 
The wood of the sycamores is diffuse porous, that is, the pores 
or vessels are of approximately the same size and more or less evenly 
distributed throughout the annual ring of growth. They are often 
grouped, and together they constitute about one-half of the trans- 
verse area between the pith rays. In outline these pores are 
irregular and may be oval, elliptical, or nearly round; the sides 
in contact with other vessels are usually much flattened. The 
vessels first formed in the spring are usually compressed tangen- 
tially. Average diameters were computed from 50 measurements 
on each of the 3 species, and show very little variance (table II). 
TABLE II 
AVERAGE, MAXIMUM, AND MINIMUM DIAMETER OF VESSEL SEGMENTS OF 
THE THREE SPECIES 
Species | Average Maximum Minimum 
Platanus occidentalis....... 0.083 mm.| o.101mm.| 0.063 mm 
Platanus Wrightii.......... 0.076 0.094 0.039 
Platanus racemosa......... 0.073 0.093 0.062 
The vessels are thin-walled and are composed of numer- 
ous short segments placed end to end. The upper and lower 
ends of these segments are usually slanting, the oblique end 
always facing the pith rays. In tangential or radial section 
these segments are readily measured under the microscope; 
table III gives averages computed from 25 measurements on 
each species. 
