(i) Annual rings:
The vascular cambium remams active throughout the life of plant, but its activity gets affected by seasonal variations. The cambial activity varies greatly in those parts of the world where the changes of climate are muchpronounced.
In autumn, when the plants do not require active translocation of nutrients and ascent of sap. the xylem elements remain compact and develop thick wall with narrow lumen. The band of xylem, produced in this season, is called autumn wood or summer wood.
On the other hand, the plants need active translocation of nutrients and ascent of sap in spring season. The cambium grows much actively in this season and produces broad, thin-walled xylem elc-mcnts wide lumen. This band of xylem is called spring wood.
Thus, the two bands of secondary xylem, i. e., autumn wood and spring wood are produced in one year. These two bands make an annual ring. The annual rings of woody angiosperms are much distinct and one can easily determine the age of plant by counting the annual rings (It is called dendrochronolgy).
(ii) Lenticles:
The lenticel is the chief aerating structure occurs as a raised corky spot in the periderm of stem. Lenticel is usually formed below an old stoma where the cork cambium is more active and cuts a mass of loosely arranged cells, called complementary or filling cells. The complementary cells possess numerous intercellular spaces. These cells are large, colourless and lack cellular contents. The phellogen, in the region of lenticel, also has intercellular spaces.
