Question

Some mature cells even lack nuclei, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants. Would you consider these cells as ‘living’?

Answer 1

Part 1: Definition of enucleation
Some cells undergo a process of enucleation in which nucleus/ nuclei are lost at maturity as an adaptation to carry out specific functions. These cells usually lose their ability to divide, however, they are still alive for a limited lifespan.

Part 2: Examples
$$\begin{array}{ccc}Features& \text{In animals}& \text{In plants}\\ Description& \text{RBCs or erythrocytes of many mammals do not possess nuclei at maturity as they have to accommodate a large number of molecules of hemoglobin. Because of an adaptation for the aerobic mode of life, their main function is to transport the oxygen and carbon dioxide in the blood which is accomplished by hemoglobin molecules.}& \text{Sieve tube cells of the vascular plants are closely associated with the companion cells through the cytoplasmic connections known as plasmodesmata. At maturity, they lose the nuclei and other cellular components and get differentiated for the translocation of food from source to sink. The nuclei of the adjacent companion cells (sister cells) control the activities of the sieve tube cells.}\\ LifeSpan& \text{Mature RBCs, as they are enucleated, do not divide; they get destroyed within 120 days of their life cycle.}& \text{The life span of sieve tube cells is variable in vascular plants. Mostly they are short-lived and function only during the season they are formed. But in few cases, they remain alive longer than a season. For example- Grape and yellow poplar = functional for 2 seasons. Basswood = functional for 5 to 10 years.}\end{array}$$ Image Part 3: Conclusion
Although the mature sieve tube cells and mammalian erythrocytic cells do not have nuclei, they are still living as they perform essential life processes.