STRUCTURE OF THE STEM. 



67 



the wheel of a wagon. The pith corre- 

 sponds to the hub, the primary niediiUary 

 rays with the spokes, the spaces between 



the spokes to the primary wood wedges, 



the felloe to the bast product, except that 

 the spokes should be seen extending 

 through it, and the tire to the periderm in 

 its various forms of development. Al- 

 though the details of tissue arraugemout 

 pertain to histology, yet the deviations 

 from the above relative positions of the 

 phloem and xylem are of such very great 

 importance in pharmacognosy that they 

 are here referred to. We may have (1) 

 the Bicollateral Bundle, in which a sec- 

 ond fascicle of phloem is placed upon the 

 inner face of the xylem, (2) the peculiari- 

 ties characterizing the monocotyledons, 

 which will be described later. 



There are three ways in which the 

 structure of the root or stem may be <'X- 



amined. 



1. A Radial Section is a longitudinal 

 section in a plane passing through the 

 centre. 



2. A Tangential Section is a longitu- 

 dinal section in a plane which does not 

 pass through the centre. 



3. A Transverse Section is one passing 

 exactly at right angles to the former two. 



The appearance presented by a radial 

 section through a perfectly developed 

 woody stem possessing open collateral 

 bundles may now be described as fol- 

 lows, enumerating the structures upon 

 either side from the centre outward, (a) 

 pith, (b) wood wedges, with medullary 

 rays, the latter, if primary, communicat- 

 ing with the pith at the centre and out- 

 ward with the cortex, and extending up- 

 ward and downward from one node to the 

 next; if secondary, extending outward 

 and inward only through the growth of a 

 single year — that is, through one annual 

 ring — and upward and downward much 

 less than the length of an internode; (c) 

 the cambium, (d) the bast bundles, sepa- 

 rated by their medullary rays; (e) the 

 phelloderm, phellogen and periderm, the 

 relations of which to one another and to 

 the bast, and the structure of which, can- 

 not be specified, owing to the extreme va- 



nny, will also dep'^nd upon the point ctf 

 development of the phellogeu and its form 

 upon the form of the hitter. 



The Eark is everything external to tlie 

 cambium. It has been proposed to 

 move the word "bark" from common lan- 

 guage, or to ignore its fixed common 

 meaning, and to convert it into a tecli- 

 nical na me for the bork. Experience 

 with English speaking people leaves no 

 hope that they will consent to give up a 

 word employed so widely and in such 

 important ways, and its technical use can 

 apparently result only in the introduction 

 of a confusion, which is more wisely 

 avoided by the coining of some new 

 name, if that of bork is seriously objec- 

 tionable, which does not appear to be the 



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 centre they 

 will be in the form of concentric rings 

 around it. Thus the centre is seen occu- 

 pied by a circle of pith, outside of which 

 is a zone of xylem or wood tissue, sepa- 

 rated by long or short medullary rays into 

 its primary and younger wood bundles. 

 Outside of the first annual ring is where 

 the intermediate or secondary bundles 

 make their first appearance- The sec- 

 ondary medullary rays (Fig. 355a) will be 

 found not to extend outward or inward 

 beyond the production of tissue of that 

 year. Instead of appearing as blades, as 

 they did in the radial section (b), the 

 medullary rays now appear as narrow 

 lines. That is, we now see the edges of 

 the blades whose sides were before seen. 

 Passing outward beyond the last of the 

 annual rings, which successively exhibit 

 a greater number of wood-bundles and 

 medullary rays, we reach the cambium- 

 ring. Outside of this we find the phloem 

 or bast bundles separated by medullary 



rays continuous with those of the wood 

 cylinder, and still outside of this the peri- 

 derm. 



The appearance of a tangential section 

 will depend, of course, upon the tissues 

 through which it passes. If it cuts the 



riation which they display in different medullary rays these will appear neither 

 stems. The composition of the bork, if as the broad sides, as at b, nor the edges 



