148 BROWN: WooDs FOR MICROSCOPIC STUDY 
cellulose, lignin, mucilage, or other components of the cell-wall, 
which appear correlated with seasonal growth. The evidence 
that such zones of chemical variation actually represent true 
growth-rings is not without structural confirmation. For example, 
on following carefully a chemical ring (Fic. 6) which appears 
sharp on its outer face, as in Xylosma hawatiense Seem., such 
evidences as the presence of short ray cells (R) or slightly flattened 
fibers (F) would seem to indicate a slight retardation of growth 
at that place. 
In case of woods in which chemical rings are present due to 
seasonal variations in lignification, such rings are readily demon- 
strated by placing cross-sections 15-20 thick in an alcoholic 
solution of phloroglucinol for two minutes, then treating with 
strong hydrochloric acid. Definite chemical rings appear, which 
are sharply defined on the outer face, blending gradually into 
deeper shades toward the interior. Similarly, the cellulose test 
may reveal zonal differences in the amount of cellulose. Another 
type of growth-ring is defined by seasonal zones of mucilage- 
reacting fibers, but such zones, though occurring in regular zonal 
alternation, may not be sharply defined on either face. In 
Alphitonia excelsa Reiss., such mucilaginous chemical rings are 
plainly visible without a lens. 
7. Liguip PENETRATION TEST 
It not jadvegnente happens that tyloses, gums, and other 
bodies which may fill the lumina of vessels and other conducting 
elements in the region of the heartwood, are displaced or lost 
during the process of sectioning or staining, so that, from a micro- 
scopic examination, one may fail to make an accurate estimate of 
their presence or abundance. The following test is of material 
aid: Alcohol is dropped upon the transverse surface of a dry 
block of wood. If tyloses, gums, or other bodies are abundant, 
the liquid spreads out over the surface; if absent, the liquid quickly 
disappears in the tissues and soon runs through to the opposite 
end. Thus, in case of the red oaks, only a few seconds are re- 
quired for the liquid to appear at the opposite end of blocks six 
inches or more in axial thickness. Woods, in general, separate 
rather definitely into two classes, those which are penetrable and 
those which are not. 
