48 COLLEGE BOTANY 



of many tissues. The ground meristem is differentiated into: 



(a) Primary cortex — a zone of cells which are mostly 

 parenchyma and lying exterior to the procambium. 



(b) Pericycle — extending from the primary cortex to 

 the phloem. 



(c) Primary medullary rays — lying between the fibro- 

 vascular bundles. 



{d) Medulla — or pith forming the central axis. 



Therefore, it will be readily seen that the prvmordial m-eri- 

 stem and the cambium,, which is, in reality, a part of it, and de- 

 rived from it, are the growing tiss'iies of the plant. The former 

 forms a cap at the tip of the shoot and the latter a cylindrical 

 sheath of actively dividing cells between the phloem and the 

 xylem. All other tissues, except those hereafter mentioned, 

 cease to grow, become fixed and are known, as fixed or permanent 

 tissues. The exceptions are the cork cambium, or phellogen, 

 from which cork is developed, and the interfascicular cambium,, 

 which unites the fascicular cambium, of the bundles, thus making 

 a complete ring. The fascicular and interfascicular cambiums 

 are iisually spoken of together as the cambium. The inter- 

 fascicular and the corh cambium are frequently referred to as 

 the second.ary cambium (Fig. 26). It will be readily seen that 

 the stem elongates by the growth and division of the cells of 

 the primA)rdial merislcm at the tip. The increase in diameter 

 is due to the growth and cell division of the cambium and sec- 

 ondary cambium, hence often called secondary meristem. All 

 other tissues soon lose the power of cell division. 



The cambium, as usually spoken of, is composed of two parts, 

 the fascicular part lying between the xylem and phloem, giving 

 rise to new elements of the bundles, and the interfascicular part. 



