THICKENING. 171 



which, however, single cells (&) appear black. These are the 

 cells in the walls of which crystals of oxalate of lime have 

 become embedded. The primary bast (v r ) is found, crushed, 

 on the outer side of that which is secondarily produced. In 

 the pericycle, after potash, far more clearly than before, single 

 irregularly-placed cells are marked by their yellow- brown con- 

 tents ; they contain resin. The cork-layer, developed from the 

 outermost layer of the pericycle, is coloured yellowish- green by 

 the potash, the thickening rings of the strengthening layer a bright 

 yellow. The endodermis has been crushed flat by the cork-layer. 



If we now examine the cross-section through a still older root, 

 about T V inch thick, which has already cast off its cortex, and 

 shows a dark-brown surface, it will show us a completely-closed 

 xylem ring ; and the appearance would hardly be distinguishable 

 from that of a cross-section of a stem of similar thickness, were it 

 not that the position of the pith is occupied by the primary plate of 

 tracheides. The surface is covered with the pericyclic cork-layer. 



Longitudinal sections through the roots are necessary in order 

 to determine that the median plate of tracheides consists of 

 exactly the same elements as the secondary wood. We again 

 find the spiral vessels at the edges of this plate, and determine 

 that the cells of the endodermis have only small height, while 

 those of the strengthening layer are far larger, and even surpass 

 in height the contiguous cells of the cortex. With aniline blue 

 we can identify the sieve-plates in the primary and secondary bast. 



Dicotyledonous roots, as a rule, have stellately-arranged poly- 

 arch wood portions, instead of the two only (diarch) which are 

 present in the Yew. In all those which thicken the same essential 

 plan is, however, carried out ; streaks of cambium appear on the 

 inner side of each portion of bast ; gradually the cambium streaks 

 extend, and join into a ring, which encloses all the primary wood 

 and excludes all the primary bast ; and forms wood internally, 

 bast externally, and medullary rays on both sides. In some 

 cases the medullary rays formed opposite to the primary bundles 

 of wood remain far broader than the others, and the ligneous mass 

 continues to be irregularly stellate, the broadening rays of this 

 mass, however, being alternate to the original rays of the wood ; 

 but as a rule these medullary rays are not distinguishable from 

 the others, and the wood is formed in rings, with cambium, and 

 outside that bast, just as it is in the diarch central cylinder of the 

 root of the Yew. 



