Ageing, death, and potential immortality lie at the heart of biology, but two seemingly incompatible paradigms coexist in different research communities and have done since the nineteenth century. The universal senescence paradigm sees senescence as inevitable in all cells. Damage accumulates. The potential immortality paradigm sees some cells as potentially immortal, especially unicellular organisms, germ cells and cancerous cells. Recent research with animal cells, yeasts and bacteria show that damaged cell constituents do in fact build up, but can be diluted by growth and cell division, especially by asymmetric cell division. By contrast, mammalian embryonic stem cells and many cancerous and ‘immortalized’ cell lines divide symmetrically, and yet replicate indefinitely. How do they acquire their potential immortality? I suggest they are rejuvenated by excreting damaged cell constituents in extracellular vesicles. If so, our understanding of cellular senescence, rejuvenation and potential immortality could be brought together in a new synthesis, which I call the cellular rejuvenation hypothesis: damaged cell constituents build up in all cells, but cells can be rejuvenated either by growth and cell division or, in ‘immortal’ cell lines, by excreting damaged cell constituents. In electronic supplementary material, appendix, I outline nine ways in which this hypothesis could be tested.

The ageing and death of cells in higher plants and higher animals are discussed in relation to cellular rejuvenation by growth and division. The full text article is available for download in the the following formats.

Death is out of fashion, rarely discussed forgotten as much as it can be. It is too close to us all. But however much or little we may choose to think about our own inevitable mortality, death is a fact of life which must be considered by any science of life. But even within biology death has been more or less ignored. I think that this has imposed a great limitation on our understanding of life itself.

Homogenates of differentiating xylem and phloem tissue have higher cellulase activities than cambial samples; the highest activity is always found in phloem. Callus tissue, in which no vascular differentiation occurs, contains only low cellulase activity. The results suggest that cellulase is involved in vascular differentiation.

Different pH optima of cellulase activity were found: in cambium, xylem and phloem tissue, cellulase activity with an optimum at about pH 5.9 is predominantly membrane-bound; it is sedimentable at 100,000 g and releasable by Triton X-100. The same may be true of activity with an optimum at pH 5.3. Phloem tissue also contains a soluble, cytoplasmic cellulase of high activity at pH 7.1 and xylem tissue contains cytoplasmic cellulase with an optimum at pH 6.5. Low cellulase activity with a pH optimum similar to that of xylem homogenates was found in xylem sap. Cellulase activity in abscission zones increases greatly just before leaf abscission. Abscission zone cellulase has two pH optima, et 5.3 and 5.9; both activities are increased by Triton treatment of homogenates. The possible existence of several different cellulases forming part of a cellulase complex, and the role of the enzymes in hydrolysing wall material during cell differentiation are discussed.

Using a viscometric method of the latex of Hevea brasiliensis was found to contain a highly active cellulase capable of hydrolysing carboxymethyl cellulose. The enzyme has a pH optimum of around 6.3. It is present in the serum of the latex and is not membrane-bound to any significant extent. Similar cellulase activities were detected in latex from old and new latex vessel rings and also in latex from regularly tapped vessels and newly tapped vessels. The possible role of the enzyme in the removal of cell wall material during the differentiation of latex vessels is discussed.

Cellulase was found to be present in the latex of species with articulated laticifers but it could not be detected in the latex of species with non-articulated laticifers. It is suggested that cellulase is involved in the removal of end walls during the differentiation of articulated laticifers.

Bleeding sap of Actinidia chinensis and Betula populifolia and guttation fluid of Avena sativa were analysed for sugars, amino-acids, auxin, and certain enzymes. A wide range of amino-acids was found in all three. Auxin was not detected in the bleeding sap, but was present in Avena guttation fluid (5.1 ug IAA equivalent/1). 'IAA oxidase', acid phosphatase, ribonuclease, deoxyribonuclease, and protease were detected in the bleeding sap and guttation fluid. The possibility that some of the substances found in sap and guttation fluid are products of autolysing, differentiating xylem cells in the roots is discussed.