PROTOPLASM AND LIFE. 745 



living cell is proved by the fact that, if the yeast be first heated to 60 

 C. and then placed in the oxygenated water, the quantity of oxygen 

 in the water remains unchanged ; in other words, the yeast ceases to 

 breathe. 



Schutzenberger has further shown that light exerts no influence on 

 the respiration of the yeast-cell that the absorption of oxygen by the 

 cell takes place in the dark exactly as in sunlight. On the other hand, 

 the influence of temperature is well marked. Respiration is almost 

 entirely arrested at temperatures below 10 C, it reaches its maximum 

 at about 40 C, while at 60 C. it again ceases. 



All this proves that the respiration of living beings is identical, 

 whether manifested in the plant or in the animal. It is essentially a 

 destructive phenomenon as much so as the burning of a piece of char- 

 coal in the open air, and, like it, is characterized by the disappearance 

 of oxygen and the formation of carbonic acid. 



One of the most valuable results of the recent careful application 

 of the experimental method of research to the life-phenomena of plants 

 is thus the complete demolition of the supposed antagonism between 

 respiration in plants and that in animals. 



I have thus endeavored to give you in a few broad outlines a sketch 

 of the nature and properties of one special modification of matter, 

 which will yield to none other in the interest which attaches to its 

 study, and in the importance of the part allocated to it in the economy 

 of nature. Did the occasion permit, I might have entered into many 

 details which I have left untouched ; but enough has been said to con- 

 vince you that in protoplasm we find the only form of matter in which 

 life can manifest itself ; and that, though the outer conditions of life 

 heat, air, water, food may all be present, protoplasm would still be 

 needed, in order that these conditions may be utilized ; in order that 

 the energy of lifeless nature may be converted into that of the count- 

 less multitudes of animal and vegetable forms which dwell upon the 

 surface of the earth or people the great depths of its seas. 



We are thus led to the conception of an essential unity in the two 

 great kingdoms of organic nature a structural unity, in the fact that 

 every living being has protoplasm as the essential matter of every liv- 

 ing element of its structure ; and a physiological unity, in the univer- 

 sal attribute of irritability which has its seat in this same protoplasm, 

 and is the prime mover of every phenomenon of life. 



We have seen how little mere form has to do with the essential 

 properties of protoplasm. This may shape itself into cells, and the 

 cells may combine into organs in ever-increasing complexity, and pro- 

 toplasm-force may be thus intensified, and, by the mechanism of or- 

 ganization, turned to the best possible account ; but we must still go 

 back to protoplasm as a naked, formless plasma if we would find 

 freed from all non-essential complications the agent to which has 



