THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 129 



From these figures it is very evident that the maximum assimilation 

 in parts of greater thickness tends towards the regions of lesser wave length. 



We come now to the second question, why it is of such special interest to 

 accurately determine the wave lengths of maximum efficiency. This is because 

 it has been frequently assumed that a connexion exists between absorption and 

 assimilation, that the maxima of absorption represented by the absorption 

 bands must at the same time represent the maxima of assimilation. We 

 have now to show that in the process of assimilation light must he absorbed, but 

 it by no means follows that this light is completely transformed locally. 

 Numerous colouring matters exhibit absorption spectra in the highest degree 

 characteristic, without necessitating in consequence the postulating of a special 

 function in the organism for the absorption bands these spectra exhibit. It is 

 well to remember that the colouring matter of the blood has a very remarkable 

 spectrum, but that it has no significance on that account in the animal economy. 

 Further, in the case of silver iodide (Ostwald, General Chemistry, II, 1070) a 

 well-marked maximum light activity appears quite near line G, although at 

 the same place, optically, absorption is quite feeble. The chief argument 

 against the coincidence of absorption and assimilation is based on the physio- 

 logical facts themselves, and it appears to us chiefly to lie in this, that attempts 

 have always been made to establish a coincidence only between the position of 

 the maximum of assimilation in the red and the absorption band near B-C, 

 perhaps (Engelmann) also a corresponding coincidence in the blue, but it has 

 never been possible to show any rise of the assimilation curve near the other 

 absorption bands of chlorophyll. Let us examine Engelmann's table (1884, 

 p. 91), wherein he shows the connexion between assimilation and absorption, 

 although it appears to us to support the opposite view. 



Wave length (/</*) . 718 680 622 589 558 522 506 486 468 431 

 Assimilation . . . 12-2 1000 80.8 60-5 47-4 39-3 59.7 66-1 59.3 459 

 Absorption . . . 23-7 8ia 52-6 47-5 402 51-0 632 83.4 863 907 



Notice especially the region X = 622-522, where assimilation falls from 80 

 to about 40, while absorption decreases only a very little and has almost the 

 same value at 522 as at 622 ; compare further A = 680, where absorption = 

 8i-2 and assimilation = 100, with \ = 431, where, in spite of more vigorous 

 absorption, assimilation reaches a value of only 45-9. 



If relations exist between absorption and assimilation these are of the 

 most complex character and have still to be worked out. This is even more 

 so in the case of the yellow, red, and blue-green cells of many Algae than of the 

 green cells of the majority of plants. In regard to these Algae, Engelmann 

 (1884) has made very interesting statements which, however, require con- 

 firmation by more extensive experimental evidence. 



It has already been made obvious that the activity of light in the assimila- 

 tion of carbon-dioxide must be bound up in a marked manner with light absorp- 

 tion. This may be assumed on theoretical grounds. The products of assimilation, 

 starch or sugar, possesses a noticeable heat of combustion, while carbon-dioxide, 

 from which they are constructed, being the end product, has no such value. 

 The energy which the plant gains in the process of constructing organic substance 

 can only originate from the sun, and hence, obviously, light, if it be altered 

 into the chemical energy of starch, must disappear as light and an absorption 

 of light must occur in the chlorophyll bodies. Julius Rob. Mayer, the dis- 

 coverer of the Law of Conservation of Energy, took up a perfectly correct 

 position with regard to the fundamental importance of this relation between 

 plants and light when he wrote (1847, pp. 37, 38) : — ' Nature has set herself the 

 task of seizing the light pouring on the earth's surface and of storing this, the 

 most mobile of all forces, in a fixed form. In order to gain this end she has 

 covered the earth with organisms capable, so long as they are alive, of absorb- 

 ing sunlight and of producing by its activity a continuous series of chemical 



