BROWN: WoOoDS FOR MICROSCOPIC STUDY 141 
Also, longitudinal sections of phloem 6 uw in thickness, when 
treated as above, will show the lattice-like arrangement of sieve 
areas in the lateral walls of the sieve tubes. 
A convenient method of numbering and labeling slides is as 
follows. By means of a camel’s hair brush, cover the entire upper 
surface of the slide outside the coverslip with dilute dammar 
(mounting consistency diluted twenty to thirty times with xylol). 
The brush is best fixed in the cork of a bottle used to contain the 
solution. In a few seconds the records may be written with Hig- 
gin’s waterproof ink, and index arrows sketched to point to any 
special part of the mount, if desired. The surface should then 
again be brushed with dammar that the slide may be freely 
handled or washed without injury to the writing. 
B. BY DIFFERENTIAL REFRACTION 
The laws of refraction may be employed not only to give clear 
definition to details in outline, but also to determine physical or 
chemical qualities of cell-walls or cell-contents. In either case 
characters may often be determined by refraction more readily 
and with greater precision than by the reaction of stains or of 
chemical reagents. Refraction opens'a wide field for research in 
plant histology and will be treated in some detail in a subsequent 
paper dealing with refraction of light in relation to plant tissues. 
6. MICROCHEMICAL REACTIONS 
Characters readily observed by microchemical means often 
have a systematic value, or are of ecological or physiological in- 
terest, or are closely identified with the peculiar qualities of color, 
hardness, hygroscopicity, porosity, durability, strength and other 
properties upon which depends the special value of any given wood 
in the arts. In the following paragraphs are indicated several 
important characters frequently found in woody tissues of warm 
climate trees, together with a number of microchemical tests 
helpful in connection with their demonstration. 
A. CELLULOSE 
Sections are placed twelve hours or longer in a solution made 
by adding one drop of concentrated aqueous Congo red to 20 c.c. 
