90 THE VEGETATIVE FUNCTIONS OF PLANTS 
be demonstrated; one can never, experimentally or other- 
wise, prove a universal negative. The principle of 
biogenesis affirms that, in experiments conducted with 
the utmost skill, and with every possible precaution to 
exclude all traces of living matter, no faintest manifesta- 
tion of life has ever been detected; we therefore logically 
conclude that, however life may have been originally 
created, it never originates now from non-living matter, 
but always from living matter only. 
88. Dissimilation. Nothing is more unstable than 
protoplasm. No sooner is new protoplasm formed by 
the process of assimilation than it begins to disintegrate, 
forming various new substances, such as cell-walls, gums, 
resins, latex, coloring matters (in flowers and other plant 
parts), the perfumes of flowers, the poisons of poisonous 
plants, and the substances that give the various flavors 
and tastes to different kinds of plants. The process by 
which all such substances are formed by protoplasm is 
called secretion. Thus we see that protoplasm is in a state 
of continual formation and disintegration. The sum total 
of all these changes, both constructive and destructive, is 
called metabolism. It is metabolism, above everything else, 
that distinguishes living from non-living matter. We also 
see that death is essential to life; unless protoplasm per- 
ishes no new protoplasm can be formed. All life, it is 
true, comes from life; but only on the condition that that 
which is already living shall perish. 
89. Economic Value of Plant Secretions. Many of 
the substances secreted by protoplasm are commercial 
products. This is notably true of wood, all of which con- 
sists of lignified cell-walls; of cork, which consists of sub- 
erized cell- walls; of the various gums, such as gum arabic, 
