io8 



PLANT LIFE. 



l^een 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 waterjjroofing 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 

 ^■j^ dicotyledonous plants; fjr ex- 

 ample, in the white water lily. 

 But the stems of the great 

 majority of dicot"\-ledonous 

 ])lants, 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. iM.-Partof a transverse section of '^"'St formed tisSUCS (fig. I 24). 

 a young stem of cinchona in process of i n^ Ciopntlfia vv mpristpm 



secondary tliicliening. /;. hairs; e. epi- ^'^^- oetOIlUdiy meilSiem. 



dermis; k, corl< cambium; lur, cortex; q-u; mnHifirnlinn nf thp cti-nft 



s, gum-resin tubes in cortex; si,, primary J '^'^ mOamcaUOn 01 tnc StlUCt- 

 phloem bundle; ^, stelar cambium; ,?, /!, r ,]^ , ■ j rhieHv 



secondary xylem; ink, pith rays; ;;/, ^-''e 01 me SttUl IS UUe CnitHy 



pith. The tissue between si and < is . .1, c^ ,- ■ , ^r .., . 



secondar>. phloem. Highly magnified.- 1° the tOl Illation Ot One Or tWO 



After Tschirch, , ,- ,. i v ■ ]■ 



layers ot actively dividing 

 cells, which (onstilute secondary mcristcm or cambium, 

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



