The initial periderm is interrupted at points around the stem by the occurrence of lenticels (Latin lentis, a lentil). These are blister-like, lenticular breaks in the surface. Most often, a lenticel on a stem forms where a stomate once occurred. The lenticel phellogen forms from cells interior to the stomate (lining the substomatal chamber) and is also connected with the adjacent cork cambium. From the lenticel phellogen cells are also produced to the outside and inside, but the outer cells tend to round up and thereby have intercellular air spaces (Phellem usually has no intercellular air spaces.), so that the tissue inside the lenticel is more loosely packed (filling tissue or complementary tissue). The cells of the lenticel also tend to expand outside the stem, yielding that blistered appearance. Each lenticel therefore becomes a pathway through which gases (especially oxygen) can diffuse to the living cells of the bark. Without sufficient oxygen, cells of bark can die.
Lenticels also can be found on fruits, e.g., the specks on apples and pears and warts on avocado.
Cells of the filling tissue are nonsuberized in some species (e.g., magnolia and cottonwood) and suberized in others (oak, elderberry, and ash). There are even species in which layers of nonsuberized cells alternate with layers of suberized cells, forming a banded pattern (e.g., cherry and birch).
Lenticels may be oriented either longitudinal or transverse. The transverse lenticels of some species (e.g., birch, jatropha, and cherry) become very elongated with age as axis circumference increases, and can exceed one centimeter in length.
Leaf scars, where leaves were attached, can be found on older stems transformed to appear superficially like lenticels.
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