GYMNOSPERMS 



327 



Cortex 

 Phloem 

 Xylem 



Leaf gap 

 Leaf base 



FIG. 191. Cross section of spruce stem 

 two years old 



The xylem is distinctive in 

 Picea ancLthe other Conifer- 

 ales (including the common 

 pines, hemlocks, and cedars) 

 by being made up almost 

 exclusively of single-celled 

 water-conducting elements 

 called tracheids (Fig. 193). 

 These single-celled trache- 

 ids resemble those of the 

 ferns and do the work of 

 the long vessels, or ducts, 

 in woody and herbaceous 

 stems of the flowering plants. The tracheids of Coniferales are 

 furnished with peculiar bordered pits representing thin places 

 in the cell wall, which later become partially roofed over by 

 the extension of the adjacent thicker portions of the tracheid 

 wall. The wood rays 

 have also certain dis- 

 tinctive features in the 

 spruce and its allies, 

 but their general struc- 

 ture and function is the 

 same as that of the 

 rays of the higher flow- 

 ering plants. The pith 

 is small and is more or 

 less irregular in outline 

 on account of the leaf 

 gaps, which, as in the 

 ferns, cause breaks, or 

 gaps, in the otherwise 



solid vascular cylinder FlG 192 Crogs gection of gpmce twig 



of the first year 



The radiate form of the pith is .due to the numerous 

 leaf gaps at the bases of leaf traces, or vascular 

 bundles, going out to the leaves. The irregularities 



--Leaf base 



of a young stem. 



The general relation 

 of the leaf traces and 

 leaf gaps to the vascular 



in the outer cortex are due to the effect of leaf bases 



