PINE STEM 285 



the bud by intervening parenchyma : here the ring has 

 been completed by the formation of an interfascicular 

 cambium in the parenchyma between the original 

 bundles. (Compare p. 99.) 



Observe that the internal limit of the vascular ring 

 is sinuous : the convexities mark the position of the 

 primary bundles : at the inner limit of these will be 

 found the protoxylem. 



4. The pith consists of parenchyma, having the 

 same characters as in the bud : there are no resin- 

 passages. 



Put on a high power, and examine the cambium. 

 Note 



i. That the cells are arranged with great regularity in 

 radial rows. 



ii. That their walls are thinner than those of the 

 surrounding tissues, and are composed of cellulose (blue 

 with chlor-zinc-iodine). 



iii. That the tangential walls are thinner than the 

 radial. 



iv. That the cells have copious protoplasm, in which 

 a nucleus may often be recognized. 



These facts point to a repeated division of cells by 

 tangential walls. (Compare Fig. 9, A, p. 99.) 



Draw carefully, and compare several of the radial series of cells 

 of the cambium. They will be found to coincide with Sanio's 

 law of cambial division, which was first concluded from observa- 

 tions on Pinus sylvestris. 



Observe, here and there, radial rows of which the 

 cells are more elongated in a radial direction than the 

 rest : these may be traced outwards towards the cortex 



