264 A TEXTBOOK OF BOTAXY [Ch. V, 10 



three forming vascular bundles. These bundles unite 

 frequently with one another into a cj-lindrical fibro-vascular 

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

 tinct morphological system called the central cylinder, or 

 STELE. This cylinder runs continuous!}^ throughout stcnis 

 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, primarj-, 

 secondary, and general. Primary growth recjuires 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 prmiary in buds and 

 root tips, and secondarjr in cambium ; but meristem itself 

 can arise anew in j'oung tissues, as in case of cork cambium, 

 the growth laj-er of some endogenous stems (page 128j, and 

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

 however, confined 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 tlie plant (Fig. 

 182). 



Epidermis, cortex, and central cjdinder, comprising tissues 

 so fundamentally important in the life of the tj-pical higher 

 plants, have been repeated so long in evolutionary history 

 that they have become fixed as morphological systems which 

 are now regular!}' laid down in distinct tissue layers b}- the 

 primary meristem of butls 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 plerorae comprises a cylintier of fun- 

 damental or parenchymatous tissue wherein are embedded 



