108 



BIOLOGY 



and cortex. They are mostly long, narrow cells with com- 

 paratively thick walls, which are hardened by the deposition 

 of woody substance. The name fibrovascular is appropriately 

 applied, since they are principally made up of fibers mixed 

 with vessels. The strength of a stem depends upon the density 

 of these bundles, and the thickness of the walls of the tracheids. 

 Of all this mass of cells only a few are filled with living 

 protoplasm. The cambium cells are always alive and the sieve 

 cells may contain protoplasm. The other cells contain proto- 

 plasm when they first form, but when they are fully grown 

 most of them are only the empty cell walls. This is particularly 

 true of the wood cells of the xylem. Protoplasm is more usually 

 found in the phloem and the cortex than in the true wood. 



Arrangement of Bundles in an Older Stem. An examination 

 of a slightly older stem shows that the bundles increase in 



width and finally fuse. 

 In Figure 48 it will be 

 particularly noticed 

 that the cambium layer 

 of one bundle has grown 

 until it comes in con- 

 tact with the cambium 

 layer of the next, and 

 thus forms a cambium 

 ring extending around 

 the stem a short dis- 

 tance within the cortex 

 separating the outer 

 portion of the stem, 



which is now called the phloem or bark, from the inner part, 

 the xylem, or wood proper. Later the other parts of the 

 bundles fuse, forming a complete ring of woody tissue and 

 a complete ring of bark separated by the cambium. 



Remembering that this cambium layer is made up of actively 

 growing cells, it is easy to see how a stem of this kind may 



fo c co 



FIG. 48. CROSS SECTION OF AN OLDER 

 STEM, SHOWING CAMBIUM FUSED TO FORM 

 A COMPLETE RING, C 



(In other respects as in Fig. 43.) 



