FEROS TT SOF WE 
6 BULLETIN 1185, U. S. DEPARTMENT OF AGRICULTURE. 
cambium layer and is not much affected by the solvent action of the 
hot water. It is not entirely because of a difference in accessibility 
that the cortex is retted after the cambium layer, as at the exposed 
surfaces of cut stems where all tissues are equally exposed to the 
attack of the bacteria, retting 1s not materially hastened in the cortex. 
It is true, however, that the cambium layer, located on the inside 
of the stem, is somewhat more accessible to the attack of the bac- 
teria than the tissues lying to the outside in the cortex. The stems 
= Pieris * 
CELL HALLS OF 11/, CUTIN INTERCELLULAR. XYLEM oR 
FIBER CELLS VA IALAYER GU/t OF F/BER BUNOLE WOOD CELLS 
lig. 2.—Cross section of flax stems, showing the consecutive order of the retting 
processes. The numbers indicate the order in which the tissue is destroyed by the 
bacteria. 1, The cambium layer; the cortex becomes loosened from the central wooden 
core. 2, The phloem parenchyma is disintegrated, exposing the insides of the bundles 
> 
to the bacteria. 38, The parenchyma between the bundles is destroyed, separating the 
edges of the bundles and exposing the outer parenchyma to contact with the retting 
liquid. 4, Most of the outer parenchyma goes next, leaving the fiber bundles held 
together by the thick-walled epidermis. 5, The epidermis and the remainder of the 
outer parenchyma and the fiber bundles are now isolated. Note that the thickness 
of the intercellular gum between the fiber cells is greater than that of the cell walls 
in the adjacent parenchyma. (Camera-lucida drawing, enlarged 480 times.) 
are partially protected from the outside by the waterproof nature of 
the cutin in the outer wall of the epidermis. The areas where the 
leaves drop off are waterproofed by the formation of leaf scars 
where suberin is deposited. At harvest time the waterproof cover- 
ing on the outside of the stem is complete with the exception of the 
stomatal openings. Part of the stomata, as microscopic examination 
shows, are closed and made water-tight by the formation of cork 
cambium. The retting liquid and bacteria, however, can find their 
way through the epidermis, as the flax stems which have been paraf- 
fined at both ends will ret. The closing up of the ends, however, does 
