12 



Im 



Elementary Plant Physiology. 



ec 



tm 



I 



Fig. 3.— Diagrams show- 

 ing method of growth and 

 formation of secondary tis- 

 sues in dicotyledonous stems. 

 A^ immediately after forma- 

 tion of generative tissues ; 

 B^ at the close of the fhrst 

 season ; C, at the close of the 

 second season ; ec^ primary 

 cortex ; /?«, limit of the cen- 

 tral cylinder ; h^ primary 

 xylem ; /, primary phloem ; 

 711^ pith ; ag^ cambium ; /j,, 

 secondary phloem at the 

 close of the first year ; h^^ 

 secondary phloem at the 

 close of the second year ; bs^, . 

 secondary xylem at the close 

 of the first year ; ^jg, secondary xylem a^^the end of the second vear. After Bon 

 nier and Leclerc du Sablon. ^ 



heavy walls, with some thin-walled elements. The outer, or 

 external portion of the cylinder contains a larger proportion 

 of cells with thin walls, sieve cells, and some fibrous or spindle- 

 shaped cells constitutiijgJJaej2hloem_aiuL^ (Fig. 3, B). 

 It is to be seen from a comparison of the two sections that 

 the region of greatest growth lies in a portion of the stem in 

 w hich all^ of^ the cells are living ^nd are cap able of division 

 and-£ xtension . In the older part of the stem, from which the 



