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 

 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 

 iun of first formed tissues (fig. 124). 

 S^Xc^tg h ° n Atf^Tep1- 134 - Secondary meristem.- 



SftA strSft xiKSS This modification of the struct- 



^^r^r^V^Z^r;;,: "re of the stem is due chiefly 



SS^i&S. ^SSHSLH to the formation of one or two 



AftcrTsd,iri1 ' layers of actively dividing 



cells, which constitute secondary meristem or cambium, 



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



