DIRECTIONS OF SECTION I-SG FOR EXAMINATION 147 



Variations in Structure.— Altliouj,^!) the details of tissue-arrangement 

 pertain to histology, yet the deviations from the above relative positions 

 of the phloem and xylem are of such very great importance in pharmac- 

 ognosy that they are here referred to. We may have (1) the Bieollateral 

 Bundle, in which a second fascicle of phloem is placed upon the inner 

 face of the xylem; (2) the peculiarities characterizing the monoco- 

 tyledons, which will be described later. 



There are three ways in which the strncture of the root or stem may 

 be examined. 



Directions of Sectioning for Examination. — 1. A Radial section is a 

 longitudinal section in a plane passing through the center. 



2. A Tangential section is a longitudinal section in a ])lanc which 

 does not pass through the center. 



\\. A Transverse section is one i)assing exactly at right angles to 

 the former two. 



Appearance of the Radial Section. — The appearance presented by a 

 radial section through a perfectly developed woody stem possessing 

 open collateral btmdles may now be described as follows, enumerating 

 the structures upon either side from the center outward: (1) Pith; 

 (2) wood wedges, with medullary rays, the latter, if primarj^, communi- 

 cating with the pith at the center and outward with the cortex; if 

 secondary, extending outward like the primary, but no farther inward 

 than the limit of the ring in which it originates; (3) the cambium; 

 (4) the bast bundles, separated by their medullary rays; (5) the phello- 

 derm, phellogen, and periderm, the relations of which to one another 

 and to the bast, and the structure of which, cannot be specified, owing 

 to the extreme variation which they dis])lay in different stems. The 

 composition of the bork, if any, will also depend upon the point of 

 development of the phellogen and its form upon the form of the latter. 



Appearance of the Transterse Section. — Upon a transverse section, 

 the same structures as above recorded will appear, but instead of being 

 in the form of thin strips upon either side of the center, they will be in 

 the form of concentric rings around it. Thus the center is seen occupied 

 by a circle of pith, outside of which is a zone of xylem or wood tissue, 

 separated by longer or shorter medullary ra\s into its primary and 

 younger wood bundles. Outside of the first ainnial ring is where the 

 intermediate or secondary bundles make their first appearance. The 

 secondary medullary rays (Fig. 42)^, a) will be found not to extend 

 inward beyond the production of tissue of that year. Instead of appear- 

 ing as blades, as they did in the radial section (Fig. 42;>, b), the medullary 



