OP VITAL ACTIONS IN GENERAL. 37 



and combining ; and in their turn they pass through a corresponding 

 series of changes ; and at length produce a new generation of similar 

 molecules, by which the race is destined to be continued. Notwith- 

 standing the mystery which has been supposed to attach itself to this 

 process, it is obvious that there is nothing in reality more difficult to 

 understand in the fact, that the parent-cell organizes and vitalizes jthe 

 protoplasma which it has elaborated, so that it should form the germ of 

 a new individual possessing similar properties with itself; than in its 

 incorporating the same material with its own structure, and causing it 

 to take a share in its own actions. 



39. Finally, the parent-cell having arrived at its full development, 

 having passed through the whole series of changes which is character- 

 istic of the species, and having prepared the germs by which the race 

 is to be propagated, dies and decays ; that is, all those operations, 

 which distinguish living organized structures from inert matter, cease 

 to be performed ; and it is given up to the influence of chemical forces 

 only, which speedily occasion a separation of its elements, and cause 

 them to return to their original forms, namely, water, carbonic acid, 

 and ammonia. It is not, however, in the dead organism alone that this 

 decomposition occurs ; for it is certain that interstitial death and decay 

 are incessantly taking place during the whole life of the being ; and that 

 the maintenance of its healthy or normal condition depends upon the 

 constant removal of the products of that decay, and upon their continual 

 replacement. If, on the one hand, those products be retained, they act 

 in the manner of poisons ; being quite as injurious to the welfare of the 

 body, as the most deleterious substances introduced from without. On 

 the other hand, if they be duly carried off, but be not replaced, the 

 conditions essential to vital action are not fulfilled, and the death of the 

 organism must be the result. 



40. Now it is to be observed that, as Plants obtain the chief materials 

 of their growth from water and carbonic acid, taking the carbon from 

 the latter and setting free the oxygen, so do they require, as the condi- 

 tion for their decay , the presence of oxygen, which may reunite with the 

 carbon that is to be given back to the atmosphere. If secluded from 

 this, the vegetable tissues may be preserved for a long time without 

 decomposition. Generally speaking, indeed, they are not prone to rapid 

 decay, except at a high temperature ; and hence it is that we have so 

 little evidence, in Plants, of the constant interstitial change, of which 

 mention has just been made. Its existence, however (at least in all the 

 softer portions of the structure), is made evident by the fact, that a con- 

 tinual extrication of carbonic acid takes place, to an amount which 

 sometimes nearly equals that of the carbonic acid decomposed, and of 

 the oxygen set free, in the act of Nutrition ( 28). The latter operation 

 is only effected under the stimulus of sunlight ; the former is constantly 

 going on, by day and by night, in sunshine and in shade ; and if it be 

 impeded or prevented by want of a due supply of oxygen, the plant 

 speedily Becomes unhealthy. Now this extrication of the products of 

 interstitial decay is termed Excretion. It is usually confined in Plants 

 to the formation of carbonic acid and water, by the union of the particles 

 of their tissues with the oxygen of the air, a process identical with 



