THE PLANT BODY 77 



the outer cylinder. The latter forms the surface of the prim- 

 ary cylinder, or the outer layer of cells of the plant. These 

 three cylinders comprise the primary tissue systems. 



The central cylinder, known as the STELE, runs continu- 

 ously throughout root and stem, and sends bifurcations into 

 the branches by which certain of its elements enter the 

 leaves to form the VEINS. It provides the PITH, or primary 

 axial tissue, and the VASCULAR BUNDLES. The latter include 

 the food-conducting tissue (PHLOEM), the water-conducting 

 tissue (XYLEM) , and between them the actively growing tissue 



- WALL 



-PLASTID 

 --SAF^CAVITY 



NUCLEUS 

 J NUCLEOLU9 

 - CYTOPLASM 



FIG. 42. Optical section (highly magnified) of a generalized plant cell. 

 (From Ganong.) 



(CAMBIUM). The cambium becomes continuous with the 

 tissues at the growing points of stem and roots, and together 

 these embryonic tissues, called MERISTEM, may be regarded 

 as the growth system of the plant. 



The hollow cylinder immediately surrounding the solid 

 central cylinder comprises the CORTICAL system which pro- 

 vides the CHLORENCHYMA, or chlorophyll -bearing tissue of the 

 young stem and of the leaves, and also the CORTEX of bark 

 and root. 



The outside cylinder forms the DERMAL system which 

 supplies the hair layer of the surface of the young root and 

 the protective EPIDERMIS covering the stem and leaf. 



