THE CAMBIUM 



65.3 



pronounced in winter, i.e. during the resting season, than it is in 

 summer. It is unlikely, however, that the decrease in the thickness 

 of the radial walls which occurs when growth and division are resumed, 

 is due to partial absorption of the thickened membranes. On the 

 contrary, both Strasburger and Kriiger are disposed to attribute this 

 change to an extension of the walls, which 

 constitutes the first indication of a resumption 

 of radial elongation on the part of the cambial 

 elements. The contents of cambial cells com- 

 prise finely granular cytoplasm and a nucleus, 

 which is generally spindle-shaped. 



It has already been mentioned on several 

 occasions, that the cells of the cambial cylinder 

 are arranged in regular radial rows. It is 

 obvious, however, that all the elements in a 

 given row are not exactly equivalent ; for 

 towards both ends of the series, the cambial 

 cells become converted into elements of per- 

 manent tissue. Sanio accordingly assumes, that 

 every row of the cambium (in Pinus sylvcstris) 

 contains a single, approximately median initial 

 cell ; each tangential division of this cell re- 

 sults in the formation of a new initial cell and 

 of a " tissue mother-cell," cut off on the outer 

 or inner side, according to circumstances. As 

 a rule, each tissue mother-cell divides once 

 more tangentially, the resulting daughter-cells 

 becoming converted into elements of perman- 

 ent tissue without further division (Fig. 272). 



According to Sanio's view, the cambium proper comprises the single 

 layer made up of the cells which are acting as initials at any given 

 moment. But Baatz and Mischke have shown, that when the secondary 

 growth in thickness is at all active, the primary segments cut off from 

 the initial cells divide again more than once. Hence it is more 

 convenient to consider the cambium as including all the cells in the 

 radial rows, which are still undergoing tangential division. According 

 to Baatz's conception, therefore, the cambial cylinder is a meristematic 

 zone, in which the number of elements in each radial row is continually 

 increased by successive tangential divisions. This extended definition 

 of the cambium is, however, by no means incompatible with the 

 existence of the initial cells postulated by Sanio. As a matter of fact, 

 Sanio's conception has recently been revived by Schoute. 



The cambium is gradually pushed outwards in the radial direction 



Portion of a radial L.S. through 

 a twig of Cytisus Laburnum, 

 showing the cambium and the 

 secondary tissues immediately 

 adjacent thereto ; p, conducting 

 parenchyma of the secondary 

 phloem ; s, young sieve-tube ; 

 c, cambium and the layers newly 

 cut off from it ; / and I, fibrous 

 elements of the secondary xylcm. 



