654 



SECONDARY GROWTH IN THICKNESS 



by the tissues which it produces on its inner side ; its circumference 

 must therefore continually increase as secondary thickening progresses. 

 Up to a certain point, the individual cambial cells can keep pace with 



this increase in the circumference of the 

 layer by means of tangential expansion ; 

 sooner or later, however, radial walls are 

 interpolated, with the result that the number 

 ^lf~ \\ of radial cell-rows is increased. Since the 



ratio between the circumference of a circle 

 and its diameter is constant ( = ir), the 

 amount of tangential extension during any 

 given interval of time can be readily 

 deduced from the radial increase in thickness 

 during the same time ; in other words it is 

 possible, in any given case, to calculate 

 mathematically how many tangential divi- 

 sions may take place in a radial row of the 

 ^ f cambium, before the interpolation of a fresh 



radial wall becomes necessary. Nageli was 



the first to perform calculations of this kind. 



The secondary permanent tissues pro- 



a'f~ ~"U duced by the cambium in Gymnosperms and 



\ IC Dicotyledons may be classified as follows. 



The intra-cambial secondary tissue, consisting 

 principally of mechanical and water-conduct- 

 ing elements, together with hadrome-par- 

 enchyma, may collectively be termed the 

 secondary xylem or the woody cylinder ; the 

 extra-cambial increment, on the other hand, 

 which is chiefly made up of leptome and 

 conducting-parenchyma often accompanied 

 by mechanical strands serving for local pro- 

 tection constitutes the secondary phloem. 

 Both sets of secondary tissues are traversed 

 by continuous radial strips of non- vascular 

 tissue, known as medullary rays. That 

 portion of a medullary ray which lies in the woody cylinder may 

 be termed the " xylem-ray," the extra-cambial portion being dis- 

 tinguished as the " phloem-ray." 



Additional or secondary medullary rays originate from one or more 

 cambial cells, which undergo a definite number of transverse and 

 oblique divisions, giving rise in this way to the initials of the several 

 cell-rows of the ray. The author has followed out the development of 



Fjg. 272. 



Small part of a T.S. through the 

 cambial zone of Finns sylvestris, 

 showing a cambium-initial and the 

 radial row of secondary elements de- 

 rived therefrom ; i, the cell assumed 

 to be the actual cambium-initial. //. 

 indicates the intra-cambial or xylem 

 portion of the radial row. x<550. 

 After Sanio (from De Bary, Comp. 

 Anat,). 



