ANOMALOUS TYPES OF SECONDARY THICKENING 133 



In many members of the Spinach-family (Chenopodiaccit) 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 xjdem 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 [Dracana) , 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 ; Co., 

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

 primary bundle ; S. 6. .secondary 

 bundle ; Xy., xylem. 



