CASPARIAN STRIPS: The endodermis have deposition of suberin or lignin on the radial and tangential walls . These thick walled cells form a continuous ring which is interrupted at certain places by passage cells.
ANNUAL RINGS: Vascular cambium remain active throughout the year but its activity gets affected by seasonal variations.
In autumn when the plant do not require active translocation and ascent of sap the xylem elements remain compact & develop thick wall with narrow lumen ,The secondary xylem band produced is called autumn or summer wood.
In spring plants need active translocation and ascent of sap the xylem elements produced in this season are thin walled with wide lumen This xylem band is called spring wood.
Thus two bands of xylem are produced in one year these two bands collectively called annual ring. These annual rings help to determine the age of a plant. Counting the annual ring to determine the age of a plant is called dendrochronology.
HEART WOOD AND SAP WOOD
In most of the plants the central older part of xylem which include the tracheids and vessel become non functional because they get filled with gums and resins produced by neighbouring parenchymatous cells and also by tylosis ( ingrowths from parenchymatous cells ) that block the xylem , this central portion of non functional xylem is called heart wood. The conduction of water takes place by young xylem cells . This outer young functional part of xylem is called sap wood.
Heart wood is less susceptible to attack of pathogen and insects & it is heavier and more durable.
SOFT WOOD OR NON POROUS WOOD: Gymnosperm wood is soft wood it is without vessels ang easy to work with.
HARD WOOD OR POROUS WOOD:
Angiospermic wood is hard wood with vessels.
LENTICELS: It is the chief aerating structure occur as a raised corky spot in periderm of stem.