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Fig. 17.3-2a and b. Transverse section of Aristolochia. The lenticels in stem bark of Aristolochia are large, with abundant amounts of complementary cells. Especially important, these lenticels have closing layers layers in which the cork cells form a somewhat tougher sheet that holds the complementary cells in place. In the low magnification micrograph, the upper arrow shows a mature closing layer and the two lower arrows indicate a closing layer that is still developing.
           
The high magnification micrograph shows part of the lenticel in greater detail, but also note two other features that it shows:

            1. The cork cambium is arising from the epidermis in this area (if your lab has slides of Aristolochia like this, look around the slide carefully and you will find areas where the cork cambium arises just below (interior to) the epidermis).
            2. The lenticel is forming before any other type of bark, before layers of ordinary cork have been produced anywhere (this is not uncommon the little tan dots on apples (the fruits we eat, not the tree trunks) are lenticels that form while the rest of the apple has just epidermis).