BROWN: Woops FOR MICROSCOPIC STUDY 145 
finished mount. With phloroglucinol and hydrochloric acid, a 
more or less pronounced red reaction is usually obtained; with 
zinc chlor-ioidide, a yellow, brownish, or sometimes purplish 
color. 
In young tissue (twigs, sapwood), the mucilaginous membranes 
stain deeply with haematoxylin, or Congo red; with zinc chlor- 
iodide they turn deep purple or violet. 
c. Fracture-—In maceration, mucilaginous fibers are usually 
extremely brittle. A fragment ordinarily shows conchoidal 
fracture across the mucilaginous core. Woods in which mucilagi- 
nous fibers are abundant are likely to be brittle. 

50 100 py 
Fic. 5. Abbé camera drawings of vessel in Tabebuia chrysantha Nichols, (?), show- 
ing surface of insoluble gum (mucilaginous) plate (M). A, after dehydration; B, 
after admission of water beneath coverslip. The shrinkage crack (C) closes in B. 
Mucilage or insoluble gum in cells, vessels, or canals.—The 
mucilaginous or gummy substances, which occasionally fill cer- 
tain cells, vessels or intercellular cavities in dicotyledonous 
woods, respond to water tests in much the same way as the 
mucilaginous portion of mucilaginous fibers. Here the substance 
does not ordinarily pull away from the sides of the cavity or canal 
on dehydration, but cleaves apart in long wide gaps extending, 
usually, across the center. Such shrinkage cracks may be observed 
to close up tightly soon after admitting water at the edge of the 
coverslip, and drawing it through by means of blotting paper at 
the opposite side (Fic. 5). The water test serves also to distin- 
guish the gums or mucilages from the resins, which, though they 
may shrink on dehydration, do not swell in water. 
