184 PLANT PHYSIOLOGY 



built together and enclosing many other compounds, simple 

 or complex, we have the ground- work for a rational concep- 

 tion of the sensitiveness of living organisms to their sur- 

 roundings, that is, of their irritability. 



A solid mass of the metals and jewels ordinarily employed 

 in the construction of a chronometer may be subjected to 

 much harsher treatment without danger of destruction than 

 the same weight of the same substances arranged as a 

 chronometer. Though the substances are the same, their 

 arrangement in the two cases is the reason for their sensi- 

 tiveness, or their power of resisting violence. Living sub- 

 stance, protoplasm, is a structure infinitely finer and hence 

 more delicate than a chronometer. Furthermore, proto- 

 plasm is composed of many more chemical compounds than 

 those entering into the structure of a chronometer. The 

 molecules of each of these compounds are composed of so 

 many atoms of so many different elements that they are far 

 less coherent and stable than the simple one, or two, or 

 three atomed molecules forming the substances in a chro- 

 nometer. Besides all this, these large numbers of atoms are 

 combined into molecules, these complex molecules are ar- 

 ranged in groups, these groups enclose and are surrounded 

 by water, and the water holds in solution oxygen and 

 a variety of compounds. The component atoms of these 

 compounds have affinities for other atoms as well as for 

 those with which they are combined. Furthermore, the sub- 

 stances may not all be in the molecular state in the solu- 

 tion; the component atoms of some substances may be 

 more or less completely dissociated. * In this condition they 

 will be still more susceptible to physical and chemical influ- 

 ences than if combined into molecules, and they will affect 

 the protoplasm with correspondingly greater promptness. 

 The intimate contact of the aqueous solution, the cell-sap, 

 with the living protoplasm, and the complete distribution of 



*See Ostwald, W. Outlines of general chemistry, Eng. transl. by 

 Walker, London and New York, 1895. Nernst, W. Theoretical chem- 

 istry, Eng. transl. by Palmer, London and New York, 1895. Jones. 

 H. C. The theory of electrolytic dissociation and some of its appli- 

 cations. New York, 1900. 



