35] 



CHAPTER II. THE TISSUES. 



137 



Begonia prolifera, underground shoots of Psilotum), or from the 

 epidermis and subjacent layers (e.g. Linaria vulgaris}. In the 

 latter case the adventitious bud arises from the pericycle (e.g. 

 Cuscuta, hypocotyl of Convolvulus arvensis). 



2. On the root. Adventitious buds may be formed either exo- 

 genously or endogenously on the root ; in the former case they arise 

 from the superficial .layers (e.g. Aristolochia Clematitis) ; in the 

 latter, from the pericycle (e.g. Alliaria ojftcinalis, Anemone 

 sylvestrisj etc.). 



B._0n the leaf. Adventitious buds developed on leaves are 

 oj exogenous origin, the epidermis being more especially concerned 

 in their production (Begonias). Adventitious roots are usually of 

 endogenous origin, being derived from cells of the pericycle. 



Adventitious buds and roots are also developed from the callus 

 (see 36) formed on the injured surfaces of stems, roots, and leaf-stalks : 

 the former may be endogenous or exogenous, the latter are endogenous. 



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 for- 

 mation of secondary 

 tissue takes place in 

 many plants, which 

 is in most cases asso- 

 ciated with a growth 

 in thickness. 



A. The Normal 

 Formation of Second- 

 ary Tissue, in_the 

 stem takes place in 

 most Gy mnosperms 

 and Dicotyledons, and is Affected by the continuous merismatic 

 activity of the cambium of their open collateral bundles. These 

 are arranged in a circle in a transverse section (Fig. 108 A) : 

 the commencement of growth in thickness is preceded by tan- 

 gential divisions in the conjunctive tissue (Fig. 105) which lies 

 between the bundles ; this gives rise to cambium 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. 108 B c), completely separating 



FIG. 108. Diagrammatic transverse sections of a 

 normal dicotyledonous stem which grows in thickness. 

 A Very young : there are five isolated bundles ; m pith ; 

 r cortex ; V primary bast ; V primary wood ; c cambium. 

 -B After growth in thickness has commenced ; Ji 2 secon- 

 dary wood ; b 2 secondary bast. 



