THE SEED PLANTS 



285 



cells (the strengtliening cells). Other parts of the leaf are the 

 stomata, which are deeply placed in the epidermis and are often 

 so clogged with dust that they are quite dark in appearance ; 



Fio. 221. Part of a cross section of a pine leaf 



The epidermis (ep), upon which is a layer of cuticle ; a layer of heavy-walled cells, 

 or sclerenchyma (set) ; stomata with deeply placed guard cells (st) ; chlorophyll- 

 beariiig tissue (cAZ) ; resin ducts, or channels <//) ; bundle sheath (bah), which in- 

 closes two vascular bundles (vb). Greatly magnified 



the chlorophyll tissue, through which run the resin ducts ; the 

 pith region, through which run two groups of fibrovascular 

 bundles, but one of which is shown in the figure. 



