60 



Growth 



The tip of the meristem in seed plants is usually a rounded, dome- 

 shaped mass of cells around the base of which the leaf primordia appear 

 in succession (Fig. 4-5). Some earlier investigators reported the presence 

 of apical cells here but later work did not confirm this. Newman (1956), 

 however, finds dividing cells in the very center of the apical dome in 

 Tropaeolum and Coleus and believes that they are to be regarded as 

 true apical cells. A similar situation has been reported in certain roots. 





\<-l 





b 4"'" •'■ ?w W>. "• • 



; ■ ' 



-„ <%~ - -\V< r- - - r t ■"..:' ••■.. v; 



fc 



"••V f 





Fig. 4-5. Longitudinal section through shoot apex of Coleus, showing meristem, leaf 

 primordia, and two bud meristems. (Courtesy Triarch Botanical Products.) 



Much attention has been paid to the structure of the dome itself. 

 Hanstein (1868) was the first to give careful study to the shoot meri- 

 stem. He noted the presence in it of well-marked layers of cells and 

 distinguished three regions, or histogens, each of which, he believed, 

 gave rise to a particular tissue or tissues of the stem. The outermost, 

 or dermatogen, is a single layer and produces the epidermis. Under this, 

 several layers thick, is the periblem, giving rise to the cortex. The inner- 

 most core, or plerome, without well-marked layers, forms the vascular 

 cylinder and pith ( Fig. 4-6 ) . 



This hypothesis would have important implications for morphogenesis 



