562 



NA TURE 



{April 14, 1 88 1 



and swell up. If the cell has been killed by prolonged 

 exposure to light, no such absorption of water will take 

 place. 



Prof. Pringsheim mentions starch, oil, and tannin as 

 commonly occurring in the chlorophyll-corpuscles ^ ; these 

 substances, which contain a considerable proportion of 

 oxygen in their molecule, are not affected bv the action of 

 light. 



3. Hypochlorin. 



No trace of this substance can be detected in cells 

 which have been exposed. 



4. The protoplasm of the cell. 



The turgidity of exposed cells is lost ; the protoplasm 

 readily allows colouring matters to pass through it, and 

 treatment with plasmolytic reagents produces no contrac- 

 tion. In a long cell, part of which has been exposed to 

 the action of light, it is only the portion of the proto- 

 plasm which has been exposed that exhibits these altera- 

 tions in its properties. Moreover, these changes are 

 apparently accompanied by a perceptible loss of sub- 

 stance. 



A brief exposure causes an arrest of the rotatory move- 

 ments of the protoplasm in cells which exhibit it. If the 

 exposure has not been prolonged, these movements will 

 recommence after a time. If a portion only of such a 

 cell be exposed for not too long a time, the movements 

 will be arrested in the exposed portion, whereas it con- 

 tinues in the rest of the cell. 



5. The cell-wall. 



The only effect which the exposure produced on cell- 

 walls appears to have been that the cells of the more 

 delicate filaments of species of Spirogyra and Mesocarpus 

 become isolated. 



The following are the general conclusions which Prof. 

 Pringsheim dra\\s from the facts which have been stated 

 above. He concludes that the injurious effects produced 

 by the exposure of a cell to intense light in an atmosphere 

 containing oxygen, are due to an increased oxidation of 

 certain substances which are essential to its life. The 

 rays which are active in this process are not those which 

 reach the interior of the cell after having passed through 

 the chlorophyll-corpuscles, but those which reach it 

 directly. He finds, it is true, the cells which do not con- 

 tain chlorophyll are, on the whole, less sensitive to the 

 action of light than those which do contain it, but there 

 are no grounds for believing that this greater sensi- 

 tiveness is due to the absorption of any light-rays by the 

 chlorophyll. On the contrary, the presence of chlorophyll 

 in a cell seems rather to diminish than to increase the 

 oxidising action of light. Facts are adduced to show 

 that chlorophyll exercises a protective influence in this 

 respect, and it is further pointed out that the death of the 

 cell, when exposed to intense light, takes place before the 

 complete decolorisation of the chlorophyll-corpuscles has 

 been effected. The greater sensitiveness to the action of 

 light of a cell containing chlorophyll is ascribed to the 

 presence in it of the readily-oxidisable products of its 

 assimilatory activity, and of these hypochlorin is the most 

 important. 



Prof. Pringsheim' s theory of the function of chloro- 

 phyll is, then, (i)that the respiration of a cell is increased 

 by exposure to light ; (2) that, in consequence of the 

 absorption which takes place when light passes through 

 chlorophyll, the presence of chlorophyll in a cell tends to 

 counteract the influence of light, so that, when the light 

 is not intense, the respiration of the cell is so far 

 diminished that processes of reduction can take place 

 within it, but when the light is very intense the chloro- 

 phyll itself undergoes decomposition, and the death of the 

 cell is brought about by the excessive oxidation of certain 

 of its essential constituents. 



' He quotes the observati,ins of Briosi to prove that oil. and not starch, is 

 the substance which is formed in the chlorophyIl-c.:)rpuscIes of the Musaceae, 

 without being aware, apparently, that Briosi's results have been shown to be 

 incorrect by Holle and Godlewski (" Flora." 1877). 



After having stated his own views, Prof Pringsheim 

 proceeds to criticise the current hypotheses concerning 

 the function of chlorophyll. The first of these is the one 

 according to which chlorophyll is itself converted into 

 carbohydrate in the assimilatory process. It is not easy 

 to understand, from a chemical point of view, how such a 

 conversion can be effected in the plant, and it becomes 

 quite impossible when it is shown, as Prof. Pringsheim 

 has done, that chlorophyll does not undergo any per- 

 ceptible change when it is exposed to' light in an atmo- 

 sphere containing carbonic acid but no oxygen. The second 

 relates to the absorption-spectrum of chlorophyll. The 

 question naturally arises as to whether or not the rays 

 which chlorophyll absorbs are those which effect the 

 reduction of carbonic acid and water in the chlorophyll- 

 corpuscle. That it must be answered in the negative is 

 clear when it is borne in mind that the maximum, of the 

 decomposition of carbonic acid does not coincide with 

 the maximum of absorption in the chlorophyll-spectrum. 

 From what source, then, is the energy derived which is 

 necessary for this reduction .' It is derived, according to 

 Prof Pringsheim, from light-rays absorbed, not by the 

 chlorophyll, but by the other cell-contents. He is of 

 opinion that the chlorophyll takes no direct part in this 

 process. 



The next question which is dealt with is the existence 

 of an optimum intensity of light for the decomposition of 

 carbonic acid. Prof. Pringsheira does not consider that 

 this is the intensity which effects the greatest evolution of 

 oxygen, for, from his point of view, the amount of oxygen 

 given off at any time is the resultant of the action of two 

 distinct processes, respiration and assimilation, both of 

 which are affected by the intensity of the light. He 

 concludes that, so far as an actual gain of carbon is 

 concerned, there is no general optimum intensity. He 

 applies this mode of reasoning also to the cjuestion of the 

 relative activity of the different rays of the spectrum in 

 the process of assimilation. The green and yellow rays 

 have been found to be more active than the blue, because. 

 as he points out, they are not absorbed to the same 

 extent by the chlorophyll, and not because they are 

 endowed with any special reducing power. Finally, he 

 proceeds to the discussion of the fact that the volume 

 of an atmosphere in which green plants are exposed to 

 light remains constant. This fact has led to the conclu- 

 sion that, since the volume of carbonic acid decomposed 

 and of oxygen exhaled is the same, the substance formed 

 in the process is a carbohydrate. Prof Pringsheim' s 

 inference from this fact is very different. He argues that 

 since oxidation and reduction are going on simultaneously, 

 the substance formed contains a smaller quantity of 

 oxygen than a carbohydrate, — in fact a quantity which 

 is smaller by just that amount of oxygen which has been 

 used up in respiration. 



Stated in the briefest possible inanner. Prof. Pringsheim's 

 principal results are as follows : — 



1. That the presence of chlorophyll favours the assimi- 

 latory activity of the chlorophyll-corpuscle in consequence 

 of the absorption, by the chlorophyll, of light, which 

 would promote the respiratory activity. 



2. That hypochlorin is the substance which is the first 

 visible product of this assimilatory activity, and that the 

 other substances (starch, glucose, oil, tannin) which 

 are found in chlorophyll-corpuscles are derived from 

 hypochlorin by oxidation. This conclusion is based upon 

 the general occurrence of hypochlorin in chlorophyll- 

 corpuscles, upon the fact that the amount of hypochlorin 

 in a corpuscle varies inversely as the quantity of starch 

 which is present, and upon the observation that hypo- 

 chlorin cannot be detected in seedlings until they have 

 been exposed to light of such an intensity as to enable 

 them to assimilate. 



It is of interest to note that hypochlorin makes its 

 appearance at the later stages of germination in seed- 



