35.] 



CHAPTER IT. THE TISSUES. 



191 



adventitious member is developed exogenously ; when the part is 

 older with its tissues more or less differentiated, the adventitious 

 member is developed endogeneously, usually from the pericycle, 

 but sometimes from still deeper tissues, according to the relative 

 age of the part. 



35. The Formation of Secondary Tissue. In addition 

 to the formation of primary tissue from the primary meristem of 

 the growing-point, as above described, a formation of secondary 

 tissue takes place in many plants, which is in most cases associated 

 with a growth in thickness and may give rise to secondary stelar 

 tissue, or to secondary extra-stelar tissue. 



A. The Normal Formation of Secondary Stelar Tissue in the 

 stem takes place in most Gymnosperms and Dicotyledons (as 

 also in some OphioglosseaB), and is effected by the continuous 

 merismatic activity of the cambium of their open collateral 

 bundles. These are 

 arranged in a circle 

 in a transverse sec- 

 tion (Fig. 143 A) : the 

 commencement of 

 growth in thickness 

 is preceded by tan- 

 gential divisions in 

 the conjunctive tissue 

 (Fig. 139) which lies 

 between the bundles ; 

 this gives rise to cam- 

 bium which becomes 

 continuous with that of the vascular bundles. A closed hollow 

 cylinder is thus formed, which appears, in a transverse section, 

 as a ring, the cambium-ring (Fig. 143 B c) completely separating 

 the pith from the cortex : it consists of two portions correspond- 

 ing to its mode of origin ; fascicular cambium, i.e. the cambium 

 belonging to the vascular bundles, and the inter -fascicular cambium, 

 i.e. that which is formed between the bundles in the primary 

 medullary rays (see Fig. 130). 



A cambium-ring is likewise formed in the root of these plants 

 (Fig. 144). The first indication of the formation of a cambium- 

 layer is the division of the cells of the conjunctive parenchyma on 

 the inner surface of each bast-bundle ; then those on the flanks 

 of the bast-bundles begin to divide ; and thus a number of arcs 



FIG. 143. Diagrammatic transverse sections of a 

 normal dicotyledonous stem which grows in thickness. 

 A Veryyonng: there are five isolated bundles; m pith; 

 r cortex; b' primary bast ; h' primary wood ; c cambium. 

 B After growth in thickness has commenced : 7i 2 secon- 

 dary wood ; b 2 secondary bast. 



