BROWN: Woops FOR MICROSCOPIC STUDY 149 
8. SUMMARY 
In cutting thin woody sections with a sliding microtome, it is 
essential, for accurate results, that careful attention be paid to 
sharpening and adjusting the knife. The blade must. be ground 
to perfect axial alignment. The wedge-shaped edge should have 
an angle of approximately 20°; for general work this gives better 
results than a more acute edge. 
In softening tissues of tropical woods for sectioning, strong 
hydrofluoric acid may be used in preference to weak. The 
length of time required to soften tissues of tropical woods varies 
from a few days to several weeks, little if any injury being done 
to the tissues by remaining a. long time in acid. The process of 
demineralization of the cell-wall may be completed without per- 
ceptible effect upon the outline of the calcium oxalate crystals 
contained in the cells, beyond the fact that the refractive properties 
may be changed. 
In the maceration of woody tissues, Schultze’s method may be 
employed with safety if equal volumes of acid and water are used. 
Fibers with one or more mucilaginous inner layers are frequent 
in the xylem of tropical trees. Perhaps the most constant proper- 
ties of the mucilaginous layers consist (1) in their marked swelling 
in water and (2) their brittleness on drying. With stains and 
reagents, reactions differ with the age of the tissue. Thus in 
young tissue (twigs, sapwood) gathered during the period of rest, 
a purple color is obtained with zinc chlor-iodide; haematoxylin, 
anilin blue, or Congo red stain deeply. In old tissue (heartwood), 
the mucilaginous layers react, as a rule, yellow or brownish with 
zine chlor-iodide, and take stains irregularly. Woods in which 
mucilaginous fibers are abundant may shrink greatly on drying 
or swell correspondingly on wetting, though not to the same degree 
as the mucilaginous membranes. 
Woods without structural growth-rings may possess chemical 
rings demonstrable by microchemical means. 
E UNIVERSITY 
LITERATURE CITED 
Chamberlain, C. J. ('15). Methods in plant histology. Ed. 3. 
Chicago. 
Guignard, L. ('92). L’appareil sécréteur des Copaifera. Bull. Soc. 
Bot. France 39: 233-260. f. I-13. 
