c. 



ANOMALOUS TYPES OF SECONDARY THICKENING 133 



In many members of the Spinach-family (Chenopodiaceae) the 

 normal cambium functions only for a short time and, after forming 

 a narrow strip of secondary xylem and phloem, ceases to divide. 

 Thereupon another cambium arises in the inner part of the 

 cortex, and a new strip of 

 xylem and phloem is produced 

 till this cambium in its turn 

 ceases to be active. This 

 process is repeated again and 

 again, each successive cam- 

 bium forming xylem on the 

 inner and phloem on the outer 

 side. In the root of the Beet 

 (Beta) the successive cambia 

 form continuous rings of xylem 

 and phloem, its fleshy char- 

 acter being mainly due to the 

 extensive development of the 

 latter tissue. As a general 

 rule, however, xylem and 

 phloem are produced only at 

 certain points, and appear as 

 secondary bundles separated 

 by the thick-walled ground- 

 tissue, developed from the re- 

 maining parts of each cambium 

 (e.g. Silver Goosefoot, Obione 

 portulacoides) . 



Another uncommon 

 method, shown by; certain 

 Monocotyledons which ex- 

 hibit marked secondary in- 

 crease in girth, is well illus- 

 trated by the Dragon-tree of 

 Teneriffe (Draccena) , the stems 

 of which may attain a diameter of ten feet or more. The bundles 

 of the young stem here exhibit the usual scattered arrangement ; 

 but in the cortex, immediately beyond the vascular region, there 

 arises a cambium (Fig. 64, Ca.) which cuts off segments mainly 



FIG. 64. Transverse section of a 

 portion of an old stem of 

 Dvaccena showing secondary 

 thickening. C., cortex ; Ca., 

 cambium; P., phloem; P.b., 

 primary bundle ; 5.6., secondary 

 bundle ; Xy., xylem. 



