74 OAK. [CH. V 



the cambium in the most diagrammatic way without being 

 seriously inaccurate. 



Fig. 34 gives, much magnified, a small portion of the 

 cambium of a pine-tree. Towards the middle of the 

 ladder-like radial row is a delicate transverse wall which 

 abuts sharply on the radial walls. This is the last 

 cell wall that has been formed, and gives evidence that 

 this is the actual region of cell manufactory or as it is 

 called the initial layer of the cambium. The cells on 

 either side of the initial layer are on their way to 

 becoming permanent tissue, and the change in form 

 which accompanies increasing age can be clearly made 

 out. In the pine-tree the xylem is made up of vessel-like 

 elements known as tracheids, and at 3 such tracheids are 

 seen cut across in transverse section ; then comes a 

 younger tracheid (2) with thinner walls, and lastly a 

 tracheid (l)with thin walls and without the "bordered" pits 1 

 characteristic of the fully developed elements. Between 

 such elements as (3) and the cells of the initial layer 

 there is a gradation of cells, intermediate in age between 

 the adult and the initial stage, and also intermediate 

 in appearance. There is a similar gradation from the 

 initial layer towards the phloem, but it is not so clearly 

 visible. 



In longitudinal (radial) sections the character men- 

 tioned above is shown in a similar way. Namely, that 

 in the radial direction cells of equal length are arranged 

 one behind the other like books in a shelf. 



A function of the cambium, which is sometimes 

 1 See the account of bordered pits in the next section. 



