In the presence of the many factors which influence the obvious diminution of cell activity in the old, it is impossible to say whether the loss of cell activity is intrinsic or extrinsic.

The life of the plant cell seems to be immortal; it does not grow old.
Trees die owing to accidents or because the tree acquires in the course of its growth a mass of tissue in which there is little or no life, and which becomes the prey of parasites.

The growing tissue of a tree is comprised in a thin layer below the bark, and the life of this may seemingly be indefinitely prolonged by placing it in a situation in which it escapes the action of accidental injuries and decay, as by grafting on young trees.

Where the nature of the dead wood is such that it is immune from parasites and decay, as in the case of the Sequoias, life seems to be indefinitely prolonged.

The growing branches of one of these trees, whose age has been estimated with seeming accuracy at six thousand years, are just as fresh and the tree produces its flowers and fruit in the same degree as a youthful brother of one thousand years.