As sieve tubes age, the callose layers become thicker, eventually blocking the SIEVE ELEMENT either seasonally or permanently.At wound and infection sites callose provides a physical barrier.

• 当筛管老化时，胼胝质层变厚并最终季候性或永久性的的阻塞筛孔，形成一个物理屏障，使筛管失去输导的功能。