222 THE POWER OF RESISTANCE TO EXTREMES 



deprived of a single essential element, but the changes and reactions which 

 render the continued existence of the protoplast impossible in the absence 

 of potassium, magnesium, or iron are quite unknown. We can, however, 

 easily see that owing to the increased metabolic activity death naturally 

 occurs in such cases sooner at high temperatures than at low ones. 



Certain injurious agencies produce a vacuolation or deformation of the 

 protoplast, but others cause no perceptible change previously to death, and 

 this is even the case in living non-nucleated fragments of cytoplasm, 

 although the absence of the nucleus must induce profound disturbances in 

 them. In spite of this, however, they may remain living for a considerable 

 time, although the disturbances due to high temperatures and the absence 

 .of oxygen produce rapid death in the intact protoplast. If the processes 

 leading to death have not gone too far, recovery may take place on the 

 restoration of normal conditions, and under such circumstances the vacuo- 

 lation or deformation of the protoplast also disappears. The same occurs 

 when the disturbance is due to a change of conditions to which the plant is 

 able to gradually accommodate itself. The removal of the nucleus acts as 

 an irremediable injury to the protoplast, whereas the effect of the removal 

 of a certain portion of the cytoplasm, and probably also of the nucleoplasm, 

 can be successfully overcome. 



Every somatic cell, and hence every adult organ, appears as far as we 

 know to have a limited duration, and hence leaves die under the most 

 favourable conditions after one or a few years, while the old parts of 

 apically growing rhizomes and mosses continually die away. The long life 

 of a tree is only attained by the continual formation of new wood and bark 

 by the cambium, and these tissues may either die in a few years or in many 

 species may remain living for a hundred years. This ultimate death would 

 probably also occur in the absence of correlative influences, for the duration 

 of the somatic parts of an annual can be somewhat, but not indefinitely, 

 increased when flowering and fruiting is prevented. 



Automatic death may probably be produced in various ways. In 

 addition to those cells in which the final stages of development lead directly 

 to death, others may exist which would be capable of unlimited life, were 

 it not that the vital activity of the cell causes it to slowly wear out and die. 

 In such cases death would not ensue if the wear and tear could be com- 

 pletely repaired, but such a perfect power of repair does not seem to be 

 possessed by the somatic cells even of the simplest vascular plants 1 . 



In asomatophytes, in which all the cells remain permanently 

 embryonic and capable of growth and division, death from internal causes 

 is avoided. Thus bacteria and yeast-cells continue to grow and divide so 



1 Frank (Krankheiten d. Pflanzen, 1895, 2. Aufl., Bd. I, p. 6) erroneously supposed that death 

 never arose from internal causes. 



