158 



GENERAL PHYSIOLOGY 



function in the construction of living substance begin to be 

 exercised. 



The process of assimilation naturally differs much according to 

 the condition of the ingested food. Differences must be recog- 

 nised also in assimilation by the two main groups of organisms, 

 plants and animals, corresponding to the differences that have 



been recognised in their food. It is 

 evident that the processes that lead 

 to the formation of living substance in 

 the plant-cell must constitute a much 

 longer series than in the animal-cell, 

 for the plant must construct the highly 

 complex proteid molecule out of the 

 simplest inorganic compounds, car- 

 bonic acid, water, salts and oxygen, 

 while the animal obtains, already 

 formed, the proteid food without which 

 it cannot live, and only needs to use 

 this in its specific manner. We will 

 follow the processes that lead to the 

 assimilation of proteids somewhat in 

 detail in the two series, so far as in 

 general they are known. The lack of 

 our knowledge is realised here as 

 elsewhere. 



To consider first the plants, a simple 

 experiment shows the first step which 

 the plant takes in the series of pro- 

 cesses that lead to assimilation. In 

 a cylindrical tube, provided with a 

 bulb closed above (Fig. 55) and gradu- 

 ated, a green leaf is placed by means 

 of a wire, and a certain measured 

 quantity of carbonic acid is allowed to 

 flow in. The lower end of the tube 

 is closed by means of mercury, and 

 the whole is allowed to stand for some 

 hours in the sunlight. If then the 

 contents of the tube be tested gaso- 

 metrically, it is found that the car- 

 bonic acid has disappeared, and in 

 place of it an equal volume of oxygen is in the tube. Since the 

 volume of the carbonic acid is equal to the volume of the oxygen 

 contained in it, the experiment proves not only that the plant has 

 taken up carbonic acid and given off oxygen, but also that it has 

 given off as much oxygen as was contained in the carbonic acid. 

 The first stage toward assimilation . in the plant is, therefore, a 



FIG. 55. Apparatus for the investiga- 

 tion of the cleavage of carbonic 

 acid in the green parts of plants. 

 (After Detmer.) 



