GYMNOSPERMS 



329 



a seed plant indicates, according to the teachings of modern 

 anatomy, that the spruces have been derived from plants with a 

 fern ancestry. In older spruce stems the leaf gaps are covered 

 over by the later-formed annual rings of wood, but they are still 

 evident as radial projections of the pith. The leaf traces are, 

 however, persistent throughout the life of the evergreen leaves 

 and may often be Leaf petiole 



seen to connect with rr. -Leaf trace 



the leaf gaps, as in 

 Fig. 194. These leaf 

 traces serve to connect 

 the phloem and xylem 

 of the vascular cylin- 

 der of the branches 

 with the green tissues 

 of the needle leaves. 

 The summary on the 

 following page gives 

 the important points 

 of similarity between 

 the anatomy of the 

 stem in the maidenhair 

 fern and in the spruce, 

 and also the general 

 advances in structure 

 made by the Oonifer- 



bium 



Leaf gaps 



FIG. 194. Gross anatomy of the stem of a spruce 

 branch two years old 



The surface of the wood cylinder is exposed in the 

 lower half of the figure by the removal of the cortex. 

 The inner portion of the wood cylinder is shown above 

 by the removal of the pith. Compare with the similar 



ales as Compared with fi S ure of the fern rhizome (Fig. 170) . Note,the breaks, 



or leaf gaps, in the wood cylinder, as in the fern 



the ptendophytes. 



The leaves of the spruce are strictly xerophytic in structure, 

 as is shown by their small size and by the thick-walled outer 

 layers of cells, which include both the epidermis and one or 

 more layers of cells beneath it. Under this hard outer cov- 

 ering of cells the green mesophyll forms a wide, cortexlike 

 layer containing chloroplastids. The central cylinder of the 

 leaf is occupied at the base by two bundles which join into 

 one in its upper portion. This vascular system of the leaf, as 

 already indicated, is a continuation of the leaf trace connecting 



