UNUSUAL . GROWTH IN THICKNESS 



59 



Some of the climbing Big- 

 noniaceas vary from the usual 

 type by the cambium failing to 

 form xylem here and there as 

 secondary thickening progresses; 

 so that the wood cylinder becomes 

 more or less deeply cleft by the 

 phloem (Fig. 30, A). In extreme 

 instances the wood may become 

 cut up into many isolated strands 

 by the production of secondary 

 meristems from cell division in 

 the wood parenchyma, medullary 

 rays and pith (Fig. 30, B), and 

 by ingrowth of cells of the peri- 

 cycle; and the secondary meri- 

 stems may add new tissues to 

 these strands and even interpolate 

 new strands among them. 



The growth in thickness of 

 fleshy roots, tubers, and rhizomes, 

 does not, as a rule, differ in 

 method from the normal type, but 

 the cambium gives rise princi- 

 pally to wood parenchyma and 

 medullary ray cells, or the cells 

 of the primary cortex and peri- 

 cycle multiply enormously and 

 constitute most of the thickening; 

 or, again, the secondary cortex 

 may be chiefly involved, as in the 

 root of Taraxacum officinale. A 

 notable exception to the normal 

 type is found in the root of the 

 beet where successive secondary 

 cambium rings are formed outside 

 the first and by their activity bring 

 about the thickening of the root. 



Fig. 30. — Diagrams showing some 

 types of unusual growth in thickness. 

 .4, cross section through a four-year-old 

 stem of Anisostichus capreolata; B, 

 cross section of stem of species of 

 Bauhinia; the xylem strands, b, are 

 stippled while the surrounding paren- 

 chyma and bark tissues are left white. 

 C, portion of a cross section of stem of 

 Gnetum scandens; i, 2 and 3 are success- 

 ive rings of growth; m, is the pith; b, is a 

 sclerenchyma ring. The xylem portions 

 with the exception of the larger tracheal 

 tubes are shaded, while the medullary 

 rays, phloem and tissues intervening 

 between the rings of growth and the 

 outer cortex tissues are left white. (.4 

 and C, after de Bary ; B, after Schleiden. 



