264 A TEXTBOOK OF BOTANY [Ch. V, 10 



three forming vascular bundles. These bundles unite 

 frequently with one another into a cylindrical fibro-vascular 

 system, which, repeated in all plants, has become a dis- 

 tinct morphological system called the central cylinder, or 

 stele. This cylinder runs continuously throughout stems 

 and roots, and into the branches (Fig. 182), but not into the 

 leaves, which receive only separate individual bundles there- 

 from ; for leaves consist simply of flattened masses of cortex, 

 with epidermis, into which extend individual bundles from 

 the central cylinder. 



6. Growth, or development of new tissues, primary, 

 secondary, and general. Primary growth requires terminal 

 position, as in buds and growing tips of roots, while secondary 

 growth requires a position among the tissues to be contin- 

 uously formed, as with cambium, which builds xylem and 

 phloem. Tissue, meristem, which is primary in buds and 

 root tips, and secondary in cambium; but meristem itself 

 can arise anew in young tissues, as in case of cork cambium, 

 the growth layer of some endogenous stems (page 128), and 

 roots like the Beet (page 257) . The growth of tissues is not, 

 however, cpnfined to meristem, but can take place in young 

 tissues by a general cell division, e.g. in expanding bark, 

 fruits, etc. Collectively the meristematic tissues form a 

 growth, or meristematic system, which, in the exogenous 

 type of structure, is continuous throughout the plant (Fig. 

 182). 



Epidermis, cortex, and central cylinder, comprising tissues 

 so fundamentally important in the life of the typical higher 

 plants, have been repeated so long in evolutionary history 

 that they have become fixed as morphological systems which 

 are now regularly laid down in distinct tissue layers by the 

 primary meristem of buds and root tips. Thus the tissue 

 layer from which the epidermis develops is called dermato- 

 gen, that of the cortex periblem, and that of the central 

 cylinder plerome. The plerome comprises a cylinder of fun- 

 damental or parenchymatous tissue wherein are embedded 



