Growth* 



n 



second section was taken, the walls of some of the elements 

 have become thickened, and the protoplasts ha ve undergone 

 dissolution. The presence of the dead cells makes impossible 

 any~elongation, excepf that permitted by the elastic stretching of 

 tfieir firm walls. Af ter th e power of elongation is lost, growth 



Fig. 4. — Diagram of longitu- 

 dinal section of a stem, tnp^ 

 apical meristem ; tnep^ epider- 

 mal meristem ; wzwz, medullary 

 meristem ; mv^ vascular meri- 

 stem ; jepy epidermis ; ec^ cor- 

 tex ; ic^ cortical parenchyma ; 

 endy endodermis ; ccy central 

 cylinder; /, pericycle ; /, 

 phloem ; b^ xylem ; ^;«, peri- 

 medullary zone ; w, medulla. 

 After Bonnier and Leclerc du 

 Sablon. 



\.^} 



in thickness may c ontinue indefinitely by the action of cells in 

 the phloem, especially of the geneiative layer, or cambium. The 

 cylinder of fibrovascular tissue and its sheath with thS enclosed 

 pith constitute a morphological unit known as a, stele ; the ar- 

 rangement of the stelar components varies in different species. 

 Cut sections from older parts of a woody stem in which elon- 

 gation has wholly ceased. Note the increased development of 



