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 first 

 season ; C at the close of the 

 second season; ec, primary 

 cortex ; Im, limit of the cen- 

 tral cylinder ; , primary 

 xylem ; /, primary phloem ; 

 m, pith ; ag, cambium ; ls^ 

 secondary phloem at 

 close of the first year ; 

 secondary phloem at 

 close of the second year ; & j, 

 secondary xylem at the close 



of the first year ; r a , secondary xylem at the end of the second year. 

 nier and Leclerc du Sablon. 



the 



the 



'*. 



Elementary Plant Physiology. 



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 constituting the phloem and cambium (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 

 which all of the cells are living, and are capable of division 

 and extension. In the older part of the stem, from which the 



