The Structure of Stems 



137 



Fig. 77. Photograph of a cross-section of 

 corn stem, showing arrangement of fibro- 

 vascular bundles. 



from the inner parts of the 

 bundles and is made up of wood 

 fibers and water-conducting tis- 

 sues. Examine a smooth piece 

 of oak and you can readily see 

 the small wood fibers and the 

 larger water tubes. You can 

 also see the pith rays that ex- 

 tend radially in thin layers at 

 right angles to the wood fibers. 



The cambium is a layer of 

 soft tissue between the two 

 sides of the bundles. It is the growing tissue which results in the 

 increase in thickness of the dicot stem. Growth takes place by the 

 longitudinal division of the cells. New cells formed on the inner 

 side by the division of the cambium layer change into water-con- 

 ducting cells or wood fibers ; on the outer side they change into 

 food-conducting cells or bast fibers. In this way the bundles of 

 perennial dicots enlarge from year to year, and this causes the 

 stem to increase in thickness. In a tree, cambium cells form a 

 continuous layer between the wood and the bark, and the di- 

 ameter is increased by the addition of successive layers of tissues 

 built by these cells. These layers are the annual rings that one 

 sees at the ends of logs. At the apex of the stem the cambium 

 terminates in the growing region. At the lower end of the stem 

 it connects with a similar tissue in the root. 



Every one who has made willow or hickory whistles has become 

 acquainted with the cambium. In early spring the cambium 

 cells are dividing actively, and the cambium layer can be broken 

 by tapping on the bark. The whole bark can then be readily 

 stripped from the wood. 



As the trunks and branches of trees age, secondary cambiums 

 arise in the cortex that develop secondary layers of cork, or hard 

 cells, or even bast fibers. These cambiums are usually irregular 



