MORPHOLOGY OF HIGHER PLANTS. 



203 



Wood made up of . 



Pith, which may be wanting. 



Xylem . 



Cambium produces 



Bark made up of 



Phloem 



Composed of vessels, wood parenchyma 

 and wood fibers ; or tracheids may 

 replace these cells, or be associated 

 with them. These are arranged in 

 groups forming radial rows which are 

 separated by medullary rays. 



Consisting of leptome and companion 



cells ; bast fibers may also be present. 



These are arranged in collateral 



groups and form, radial rows which 



I are separated by medullary rays. 



Pericambium, producing parenchyma and cork. 



Fig. 113. Transverse section of one of the collateral mestome strands of the stem of 

 Viola tricolor arvenis: o, portion of cells of pericycle; e, endodermis; 1, leptome or sieve 

 cells, in among which are some collenchymatic cells (c); m, cambium; t, spiral tracheas or 

 vessels; g, strongly lignified trachese; rp, medullary ray cells, the walls of which are com- 

 posed of cellulose; rs, medullary ray cells the walls of which are strongly lignified; s, 

 strongly lignified cells separating the mestome strands; c, collenchyma; p, pith. 



