The Substance Which Is Alive in Plants 163 



stronger and stronger and works better and better up to a certain 

 culminating level, beyond which it tends to decline, and finally 

 to cease work altogether. It is probable that the decline and 

 cessation of work is dependent upon purely physical causes, some- 

 what as a bar of metal when too often bent, becomes weakened 

 and broken at last; but in this peculiarity of protoplasm we find 

 an explanation for the cycle of youth, maturity, old age and death. 

 When, however, the protoplasm can periodically alter its location, 

 habits, or functions, — can re-melt itself, so to speak, — it renews 

 its youth thereby, and can continue its vigor without limit, thus 

 becoming potentially immortal. In this fact is found the ex- 

 planation of the benefit ^vrought by a change of scene or occupa- 

 tion, or a vacation, upon ourselves, though the effect is here 

 limited; and if a way could be found to affect our protoplasm 

 more profoundly, — to make it mix itself up periodically, even 

 within the limits of the same cell, — then, it seems likely, man 

 would have discovered the long-sought elixir of life and the 

 secret of perpetual youth. This in fact is the case in full degree 

 in simple plants like Bacteria. Each of these is made of one cell, 

 and when it reaches full size divides into two, each of which 

 grows up and divides again, and so on without limit, in perennial 

 change, vigor, and youth. A similar rejuvenation takes place in 

 sexual reproduction, when the protoplasm of two individuals 

 mingles together in fertilization. Now, the higher plants possess 

 no organs at all in which the protoplasm continues to work within 

 the same cells throughout the life of the individual, but, as our 

 chapter on growth will abundantly illustrate, the protoplasm is 

 continually moving outward and onward into newly forming 

 buds, leaves, roots, and stems; and this removal permits it to re- 

 new its youth perennially. Therefore plants should never grow 

 old from internal causes, in the way that animals do, — and in 

 fact they do not, the exception presented by annuals being only 

 apparent and not real. Even the greatest trees continue to form 

 new leaves and roots with unabated vigor until they are brought 



