THE TISSUES OF THE PINE STEM 367 



growing point before the fibro-vascular bundles and bark are 

 formed. It practically disappears as the stem grows older and 

 the wood increases by a number of years of annual growth. 



The wood, or xylem, comprises by far the greater part of older 

 stems, becoming proportionally greater as each annual ring is 

 added. It is composed of very much elongated' cells, called 

 tracheids, with firm, somewhat yellowish, thick walls. Cell 

 walls- of this character are said to be lignified. These cells con- 

 tain pits (Fig. 297, E, F, G) surrounded by a circle and termed 

 bordered pits, the circle being a feature characteristic of this 

 group of plants. There are resin ducts among the wood cells, 

 and also peculiar plates of cells called medullary rays which 

 extend through the cambium and bast into the outer wood. The 

 medullary rays have the form of thin knife blades penetrating 

 the wood for various distances. 



The cainbium is a cylinder of thin- walled dells just outside 

 of the wood, and is the most active region of growth in the 

 stem. This cylinder (Fig. 297, C) is only two or three cells 

 wide, and the cells are continually dividing by walls parallel to 

 the surface (tangentially) during the season of growth. The 

 -daughter cells on the inside of the cambium become firm wood 

 cells by the thickening of their walls together with certain 

 changes (lignification) that give them firmness ; they also become 

 empty of protoplasm. The daughter cells on the outside of the 

 cambium form the bast, remaining soft and containing proto- 

 plasm and much food material. The cambium thus adds cells to 

 the wood on the inside and the bast on the outside. The wood is 

 deposited in annual rings during the season of growth, and these 

 are sharply distinguished from one another because the wood 

 cells formed at the beginning of one season are larger than those 

 formed in the latter part of the previous season (Fig. 297, D). 



The last is difficult to study chiefly because the cells are 

 under severe pressure from the growing cambium on the inside 

 and the restraining bark on the outside, and the cell arrange- 

 ments are frequently distorted. 



