64 



ESSENTIALS OF VEGETABLE PHARMACOGNOSY. 



stitutes a Stele (all inside of c), which in which meet at the centre, are known as 

 the root is always in the form of a Cen- 

 tral Cylinder. The plerom exhibits at 

 first only slight differences ia the appear- 

 ance of its cells (Fig. 352a), and a trans- 

 verse section of it viewed with the mi- 

 croscope might be figuratively compared to become converted into continuous 

 to looking down upon a honeycomb built tubes, the Ducts, though some of them 



the Xylem-bundles, or Wood-bundles, 

 constituting the woody portion of the root. 

 In a few plants which we have to consider, 

 the Gymuosperms, no series of cells lose 

 their end walls as above described so as 



in a cylindrical tin box, the latter repre- 

 senting the endodermis, and in longitudin- 

 al section to a longitudinal section through 



connect by perforations. 



The other bundles (Fig. 354f) which 

 have been described as alternating with 



the same. Further away from the tip, the xylem, or wood-buudlos, possess as 



however, it would be found that groups 

 of its cells (Fig. 353 e and f) had elon- 

 gated in a longitudinal direction and 

 these, to coatmne our illustration, might 

 be compared to bundles of pencils or 

 quills set in the honeycomb. Mingled 

 among the elongated cells of the bundle, 

 however, are many which have, not elon- 

 gated. These bundles would be arranged 

 in a circle, separated from one another 

 by more or less of the honeycomb tissue, 

 these separating portions corresponding 

 to the Medullary Rays of the Stele (g). 

 From the endodermis they would be sep- 

 arated by one or more continuous circles 

 of the honeycomb cells, corresponding to 

 the Pericyy?le^ or "Pericambium" (h). 

 For a time there would also be left a cen- 

 tral portion (i), consisting of unchanged 

 cells. The elongated cells, which consti- 

 tute the important elements of the bun- 

 dles, are joined end to end with other 

 similar ones still further up in the older 

 part of the structure. At first the end 

 walls of these abutting cells separate their 

 cavities from one another, but later they 

 disappear in some, becoming perforated 

 in others, so that the cavities become 

 more or less continuous, forming the Ves- 

 sels, extending throughout the root and 

 into and through the stem above. The 

 bundles thus formed are seen to be of 

 two kinds, alternating in the circle. Each 

 of those of one kind (Fig. 354e) extends 

 gradually toward the centre by the suc- 

 cessive conversion of the original cells 

 left there into vessels or into cells asso- 

 ciated with the vessels of the bundles. 

 Upon meeting there, they of course cut off 

 the^ previously existing central communi- 

 cation between the medullary rays, which 

 are now left as isolated plates or wedges 

 between the bundles. These bundles 



their important element those cells which 

 become connected by perforations in the 

 form of sieves, and are known as the 

 Phloem-Bundles. Collectively they form 

 what ig known as the Sieve- tissue, or 

 Cribrose-tissue, of the plant, and their in- 

 tercommunicatiiig tubes are the Sieve- 

 tubes. This tSssue characterizes tht 

 Gymnosperms as well as the Angiosperms. 

 The phloem-bundles do not extend toward 

 the centre, as do the xylem-bundles, but 

 stand isolated, each between two medul- 

 lary rays which separate it from the 

 xylem-bundle upon either side. In con- 

 nection with the ducts, or their equiva- 

 lents in the gymnosperms, and the other 

 tissues of the xylem-bundles, develop 

 strong fibres, the wood-fibres, while in 

 connection with the sieve-tubes and other 

 tissue of the phloem-bundles usually de- 

 velop very similar fibres, the Bast-fibres. 

 The phloem-bundles therefore ordinarily 

 become Bast-bundles, Vascular bundles 

 in which fibres develop are known as 

 Fibro-vascular bundles. 



The condition now reached by the root 

 constitutes the completed primary struc- 

 ture of its stele. With the production of 

 the primary structure growth and in- 

 crease in thickness may cease (Monoco- 

 tyledons), in which case the periderm 

 changes which we have recorded will not 

 occur. On the other hand, secondary 

 growth may take place, in which case 

 those changes are more or less completely 

 induced. In such case the cells touching 

 the phloem-bundles upon their inner faces 

 and upon their sides become meristematic 

 and proceed to produce xylem-tissue upon 

 their inner faces and secondary phloem 

 npon their outer, in contact with the pri- 

 mary tissue of that kind. Each such arc 

 of meristem (Fig. 354x) becomes the Cam- 



