Growth. 13 



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

 have become thickened, and the protoplasts have undergone 

 dissolution. The presence of the dead cells makes impossible 

 any elongation, except that permitted by the elastic stretching of 

 their firm walls. After the power of elongation is lost, growth 



Fig. 4. Diagram of longitu- 

 dinal section of a stem, mp, 

 apical meristem ; mep, epider- 

 mal meristem ; mm, medullary 

 meristem ; mv, vascular meri- 

 stem ; ept epidermis ; ec, cor- 

 tex ; tc, cortical parenchyma ; 

 end) endodermis ; cc, central 

 cylinder ; /, pericycle ; /, 

 phloem ; , xylem ; pm, peri- 

 medullary zone; m, medulla. 

 After Bonnier and Leclerc du 

 Sablon. 



cp* 



ep 



in thickness may continue indefinitely by the action of cells in 

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

 cylinder of fibrovascular tissue and its sheath with the 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 



