THE GROUND MERISTEM 27 



interrupted ring where later the vascular bundles appear. In 

 cross sections the procambium cells a;re much smaller than 

 those of the surrounding ground meristem; and in longitudinal 

 sections they are seen to be elongated, and more or less pointed 

 at the ends (Fig. ii). In the evolution of a procambium strand 

 from the primordial meristem groups of vertical rows of cells 

 of the latter divide repeatedly by vertical walls, producing the 

 small cells so plainly made out in cross sections; these cells then 

 elongate, as seen in longitudinal sections (Fig. ii). They are 

 then ready to differentiate into the different elements of the 

 vascular bundles, and sometimes into bast fibers, as will later 

 be shown. 



In the stems of Monocotyledons the procambium strands are 

 scattered promiscuously instead of being disposed in a ring 

 (Fig. 27), as in Dicotyledons; and in roots the procambium 

 occurs as a single central strand. 



The Ground Meristem. — This has been derived from the 

 primordial meristem by cross, as well as vertical, divisions of 

 its cells. It is distinguished from the procambium by not being 

 particularly elongated in any dimension, and from both pro- 

 cambium and protoderm by the relative largeness of its cells and 

 the presence oftentimes of intercellular spaces. 

 , The cells of the primordial meristem retain the power of 

 division throughout the life of the plant, but those of the primary 

 meristems, namely, protoderm, procambium, and ground meri- 

 stem, after a time cease to divide, excepting the cambium cells 

 from the procambium, as will later be shown; and they then 

 form what is known as the primary permanent tissues, the pro- 

 toderm giving rise to the epidermis, the procambium forming 

 the primary vascular bundles, and the ground meristem differ- 

 entiating into the primary cortex, pericycle, primary medullary 

 rays, and pith, as already stated. 



While these permanent tissues are characterized by the cessa- 

 tion of cell division, yet in large groups of dicotyledonous plants 

 the cells of the primary cortex habitually give rise by cell division 

 to a zone of cork cambium or phellogen from which the cork is 



