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, 



