THE STEM 213 
and Gymnosperm stems it therefore includes the xylem regions 
of the bundles, the xylem portions of the medullary-rays and 
the pith, while in the roots of secondary growth of these plants it 
comprises the xylem portions of the bundles and the xylem 
medullary-rays. From the pure botanical viewpoint, wood is 
synonomous with xylem and so represents that portion of a 
vascular bundle that contains trachez or tracheids or both. 
As the cambium year after year adds new layers of wood to 
that already present on its inner face, the conveying of sap and 
storing of starch, etc. is gradually relegated to the outer wood 
layers, since the inner layers, step by step, lose their protoplasmic 
contents and power of conducting sap and become filled with 
extractive, resinous and coloring matters. The outer whitish 
layers of wood which contain living cells, functioning in the 
vegetative processes of the plant, constitute the ALBURNUM or 
Sap-woop. The drug Quassia is a good example of this kind of 
wood. ‘The inner dead colored layers constitute the DURAMEN 
or HEART-woop. Important examples of this kind of wood used 
in pharmacy are Lignum Guaiaci, Haematoxylon and Santalum 
Album. 
Ty.tosrs.—These are bladder-like enlargements of cell walls 
usually of wood parenchyma or medullary ray cells which extend 
into the lumina of adjacent trachez or tracheids through pit 
cavities in the walls of the latter. ‘They are formed by the surface 
growth of the pit membranes of the one-sided bordered pits which 
push their way into the lumina of the vessels or tracheids. In its 
living state the tylosis contains cytoplasm, cell sap, and often a 
nucleus, the latter having passed into it from the parent cell. In 
its fully developed state it may contain starch, gum, resin and 
crystals, and its wall may be wrinkled. Sometimes a single 
parenchyma cell may produce several tyloses. Tyloses occur 
both in angiospermous and coniferous woods although are 
probably more common in the former. They are more abundant 
in heartwood than in sapwood and are of constant occurrence in 
some species. Excellent examples of tyloses can be seen in the 
rhizomes of Asarum and Serpentaria and in the stems of Sassafras, 
Juglans, the locust (Robinia), Catalpa, Poplar, Sumachs, Oaks 
and White Pine. 
