68 BOTANY 



curious patterns ; with substitution of lignin for cellu- 

 lose as the material of which they are composed; and 

 with the disappearance of many of the transverse 

 separating walls, causing a vertical row of cells to be- 

 come a vessel. The cells to show the change first are 

 those on the inside of the wedge-shaped strand the 

 protoxylem. In these the thickening of the walls is 

 laid down in the shape of a spiral band, or a series of 

 rings. These vessels remain of small diameter. The 

 other wood cells and vessels are thickened more irre- 

 gularly and are called reticulated ; in some cases when 

 the thickening deposit leaves only very small thin spots 

 they are known as pitted elements (Fig. 13, p. 35). 



The bast of the strand begins to be differentiated on 

 the side nearest the periphery, where the cells are called 

 the protophloem. The vessels of the bast are sieve tubes 

 (Fig. 27) as in the root. The other elements are mainly 

 elongated cells with thin cellulose walls. 



As the differentiation begins at the front and back of 

 the bundle and advances in each direction the wood and 

 bast are not very long in meeting. In plants that only 

 live for a few weeks or months they come into actual 

 contact, but to those whose lives are longer provision is 

 made for further development by the last layer left 

 between them remaining meristematic or capable of con- 

 tinuous dividing. This is the cambium layer of which 

 we have spoken. It is only a single cell in thickness. 



This arrangement of the supporting tissue is very 

 strong and most economical. The hollow cylinder or 

 tube is one of the strongest forms of support that a 

 structure can possess. It has, too, a certain flexibility, 

 for while the strands are gradually hardening they can 

 bend freely without breaking. The young stem thus 

 shows itself built for toughness and elasticity, so pos- 

 sessing a power of bending to wind and recovering as 

 the force of the air passes it. The continuity of the 



