DEVELOPMENT BEHIND THE APEX 321 



partly developed leaves over the shoot tip gives the 

 characteristic " bud " structure. 



The surface layer of the meristem is quite separate 

 from the underlying meristematic tissue, i.e. it forms new 

 cell walls only in the direction perpendicular to the 

 surface, and thus gives rise only to the epidermis of 

 the shoot. Below this surface layer of the meristem the 

 cell divisions give rise to the young tissues of the cortex 

 and of the vascular cylinder. 



Development of the Shoot immediately behind the 

 Apex. — As in the case of the root, the products of division 

 of the meristematic cells away from the apex increase 

 in size by the development of vacuoles, and at the same 

 time divide less frequently. 



Pith. — The cells in the centre of the stem, which will 

 give rise to the pith, are generally the first to lose their 

 meristematic activity. They often increase greatly in 

 size, partly by their own growth, and partly because 

 they are passively stretched by the active growth of the 

 surrounding tissues. This very frequently results in 

 the pith cells becoming the largest parenchymatous cells 

 in the stem. Sometimes the energetic growth of the 

 surrounding tissues, especially in length, after the 

 growth of the pith cells has ceased, leads to the breaking 

 and ultimate death of the pith. This is the clause of 

 the frequent hoUowness of herbaceous stems in which 

 the whole of the pith or its central portion has disappeared 

 in the adult stem. 



Zone of the Vascular Bundles, etc. — Just outside the 

 pith, in the zone which gives rise to the vascular bundles, 

 the rays and the pericycle, very many of the cells soon 

 cease to divide by horizontal walls, divisions continuing 

 only parallel to the axis of the stem. Growth in length 

 of these cells continues, so that they become elongated. 



21 



