BAIRD: ANATOMY OF PLAT ANUS. 285 



which bears them. Just below the terminal bud there is an 

 enlargement of the stem. In this part of the stem there is an 

 extra amount of pith, containing a rich store of food for the 

 unfolding leaves of the following season. This pith is lignified 

 and plain pitted. A mass of stone cells is found at one side of 

 the stem in this region, just beneath the peduncle or just be- 

 neath the fundament of a flower bud which never develops. 



The Bark. 



There are bast fibers in the young bark, occurring usually 

 in large bundles just outside the phloem (fig. 44, 6), but scat- 

 tered fibers also occur in the same zone. These bast fibers 

 vary from .3 millimeter to 1 millimeter in length. No sec- 

 ondary bast is formed. After a time phellogen is formed 

 within the secondary phloem, and the outer bark including all 

 the bast fibers is shed, so that the bark on the older limbs con- 

 tains no bast fibers. The walls of the cells of the medullary 

 rays in the bark become much thickened and pitted. In the 

 rays groups of lignified cells alternate with groups of unligni- 

 fied cells. The part of the ray formed early in the season be- 

 comes lignified, while the part formed late remains cellulose. 

 In both young and old bark masses of lignified parenchyma 

 cells are scattered through a ground mass of cellulose paren- 

 chyma cells. The older bark contains a very large proportion 

 of stone cells (fig. 46, s) . 



The Wood, 

 The wood (fig. 52) is composed of tracheal tubes, wood 

 parenchyma, fiber tracheids, and the parenchyma of the medul- 

 lary rays. There are no wood fibers. The greater part of the 

 wood is composed of fiber tracheids. The tracheids, the shape 

 of which is shown in figure 55, have an average length of .64 

 mm. and an average width of .0178 mm. These fibers have 

 oval bordered pits placed obliquely (fig. 52, k) . The wood 

 parenchyma, of which there is little, has plain pits and lies 

 next to the tracheal tubes. The largest tracheal tubes have a 

 diameter as great as .075 mm. These tubes have oval bordered 

 pits arranged in transverse rows (fig. 52, w) . In a longi- 

 tudinal section of one of the largest of these tubes there are 

 as many as seven of these pits to the row. By performing the 

 following experiment the length of the tracheal tubes was 

 found to be 22 cm. : 



