THE SEED PLANTS 285 
cells (the strengthening 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 ; 
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Fic. 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 (scl); stomata with deeply placed guard cells (st); chlorophyll- 
bearing tissue (chl) ; resin ducts, or channels (rd) ; bundle sheath (bsh), 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. 
