Exchange of gases between the outer atmosphere and internal tissue of the stem takes place by (in dicot stem)
a)	Lenticels	b)	Stomata	c)	Hydathodes	d)	Pneumatophores

(a)

Lenticels.

At certain regions, the phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells. These parenchymatous cells soon rupture the epidermis, forming a lens-shaped openings called lenticels. Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem. These occur in most woody trees