170 



XII. STRUCTURE OF ROOTS. 



of the median plate of tracheides (') we see the inner inactive 

 layer of the conjunctive ground- tissue (/), which may be considered 

 equivalent to a pith ; farther on the newly-formed and radially- 

 arranged tracheides (t"), with numerous interpolated medullary 

 rays. 



It is easier to obtain information as to these relations if a 

 little potash solution is added to the preparation. The spiral 

 vessels (s) at the ends of the median plate stand out clearly, with 

 dark outline ; the tracheides with bordered pits (') of this median 



FIG, 68. Cross-section of a root of Taxus haccata, after the thickening of the central 

 cylinder has commenced, c, cortex; m, strengthening layer ; e, eudodermis ; j) t 

 pericycle ; s, spiral vessels ; t', primary band of tracheides ;f, bands of ground- 

 tissue ; t", secondary tracheides with medullary rays ; v", secondary bast ; v', 

 crushed primary bast ; /, cells in the secondary bast with crystals in their 

 walls ; /, resin-containing cells in the pericycle;-(x 42). 



plate, as well as those of the secondary (cambial) growth (t"), are 

 stained a beautiful yellow ; the inner connective tissue (pith) re- 

 mains colourless. The secondary xylem bands have a plano-convex 

 contour ; at their ends they taper off, but do not come near the 

 spiral vessels. On the outer margin of the secondary xylem 

 we find the cambium, and outside that the secondary bast (v")>. 

 which after treatment with potash appears colourless, but in 



