PLANT ORGANS AND ORGANISMS 
IJ 3 
and coloring matters. The outer whitish layers of wood which con- 
tain living cells, functioning in the vegetative processes of the plant, 
constitute the alburnum or sap-wood. The drug Quassia is a good 
example of this kind of wood. The inner dead colored layers con- 
stitute the duramen or heart-wood. Important examples of this 
kind of wood used in pharmacy are Lignum Guaiaci, Hasmatoxylum, 
and Santalum Album. 
Microscopic Characteristics of Angiospermous and Gymnosperm- 
ous Woods.— The wood of Angiosperms is characterized by the pres- 
ence of tracheae (vessels) with various 
markings on their walls, particularly by 
small pits in the walls of some of the 
tracheae, together with wood fibers, 
wood parenchyma and medullary rays. 
The wood of Gymnosperms is made 
up for the larger part of tracheids with 
bordered pits which latter are charac- 
terized in radial longitudinal section by 
the presence of two rings, one within 
the other. A single row of these is 
seen on the tracheid wall. Medullary 
rays, frequently diagnostic for different 
species and woody parenchyme cells, 
are also found. 
Histology of Typical Herbaceous Monocotyl Stems (Endogenous). 
— Passing from exterior toward center the following structures are 
seen: 
Fig. 51. — Photomicrograph 
of cross-section of very young 
cornstalk, where certain 
plerome strands have just 
gone over into vascular bun- 
dles. For comparison with 
Fig. 52. (Stevens.) 
1. Epidermis whose cells are cutinized in their outer walls. 
2. Hypodermis, generally collenchymatic. 
3. Cortex. 
4. Endodermis or innermost layer of cortex. 
5. A large central zone of parenchyme matrix in which are found 
scattered fibro-vascular bundles of the closed collateral or rarely 
concentric type (amphivasal). In this latter type, which is typical 
of old monocotyl stems, the xylem grows completely around phloem 
so that phloem is found in the center and xylem without and sur- 
rounding it. 
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