io8 



PLANT LIFE. 



been described, undergoing only slight changes in the char- 

 acteristics of the individual 

 tissues which compose it. 

 Thus, with age, there may be 

 a thickening of the tissues so 

 as to impart greater rigidity ; 

 or the waterproofing of the 

 exterior may be made more 

 perfect. These and similar 

 changes do not, however, 

 materially alter the structure. 

 This permanence of primary 

 structure is particularly fre- 

 quent in the stems of mono- 

 cotyledonous plants. It has 

 been observed also in some 

 m fo dicotyledonous plants; for ex- 

 ample, in the white water lily. 

 But the stems of the great 

 majority of dicotyledonous 

 plants, as well as the conifers, 

 quickly lose their primary 

 structure, adding . tissues of 

 considerable amount, so as to 

 bring about a more or less 

 striking rearrangement of the 



Fig. i 24 '.-Pa7of a transverse section of first formed tissues (fig. I2 4 ). 



t^£fS^t?°T«,t£rr^ 134 - Secondary meristem.- 

 ^S^n^s^S, ZiSS This modification of the struct- 



f ' ure of the stem is due chiefly 

 _ to the formation of one or two 



secondary xylem; mk, pith rays; 

 pith. The tissue between sb and c is 



secondary phloe 



After Tschirch. ^^ Qf actiyely dividing 



cells, which constitute secondary meristem or cambium, 

 roughly parallel to the surface. When there are two, one of 



