36 MISSOURI BOTANICAL GARDEN. 
fibers without any impurities whatever. The quantity of 
wood transformed into cellulose is exceeded only, as far as 
I know, by that found in diseased wood of Juniperus, 
decayed by Trametes Pini, to be described in another 
paper. 
With polarized light, the prisms being crossed, the pri- 
mary lamella of sound wood appears white, i. e., it is 
highly refractive; the secondary lamellae are darker. The 
rotted wood, with the exception of some very minute parti- 
cles, allows no light to pass. The hypothesis of a crystal- 
line structure of the cell-wall, as advocated by Niigeli, is 
based largely on its optical properties. Nigeli held that 
the double refraction indicated a condition of stress in more 
than one direction. The absence of any refraction in the 
rotted wood indicates a homogeneous condition, i. e., one 
in which the stress is equal in all directions. The change 
from sound wood to the decayed form must have been a 
profound one to bring about this condition. It has been 
noted for wood destroyed by Merulius lachrymans,* that it 
separates the white polarized light into blue and yellow 
parts. Hartig makes no attempt to explain why this should 
be so. In this connection it may be said that rotted wood 
of Libocedrus decurrens, yet to be described, and wood of 
Juniperus Viginiana destroyed by Polyporus carneus, 
appear dark when viewed with crossed prisms. 
Humus ComMpounn. 
In the cells immediately surrounding the rotted areas 
certain parts of the walls are colored dark brown by an 
apparently homogeneous substance. This occurs in various 
forms. Most commonly it has numerous cracks and fissures 
breaking it into many plates, looking much like mud which 
has dried in the sun (Pl. 4, fig. 4), then again it appears 
in the form of irregular granules scattered along the walls, 
* Hartig, R. Der iichte Hausschwamm 61. Berlin. 1885. 
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