SECONDARV THICKENING. NORMAL DICOTYLEDONS. 



4<53 



to this, even casual observation shows that the secondary growth on the side of the 

 ■wood is almost always far more abundant than on the side of the bast. 



Both the successively developed elements of the wood and bast, and the cells of 

 the initial layer, in certain cases, increase for a time in size, while growth in thickness 

 proceeds, as will be described in greater detail below ; from a definite period onwards, 

 however, a constant average size is assumed by all the elements which arise subse- 

 quently, while in other cases the average size remains approximately equal from the 

 beginning. 



The number of elements in the tangential zones, and .hence the number of 

 radial lows, must therefore be constantly increased as growth in thickness proceeds. 



Fig. 196. FIG. 197. 



Fig. 196. — Sambucus nigra ; young internode ; cross-section (220). P, P limit of the parenchyma of the external cortex. 

 Between P—P and r—r primary zone of bast (phloem) ; c cambial zone ; g, g (pitted) vessels in process of formation ;. fi—p 

 parenchyma of the pith. The cross-sections with a double outline, at and above s, are the spiral vessels of a leaf-trace bundle. 



Fig. 197. — The radial row x—x from the cambial layer of Fig. 196 (600) ; e seems to be the cambial initial cell, just divided : 

 k side towards tlie wood ; r side towards the cortex. 



and this takes place by means of radial divisions of the initial cells into two equi- 

 valent daughter -cells, which then perform the functions of initial cells in the manner 

 described. Assuming that the woody cylinder within the cambium undergoes no 

 further enlargement^ and that all the secondary elements successively formed are of 

 equal size, it is shown, by a simple theoretical consideration ^, that for each radial 



' Nageli, Dickenwachsthum des Stengels, &c., bei den Sapindaceen, p. 15. 



