792 EEPORT— 1902. 



or no this is the case in plants generally cannot at present be said, though it 

 appears from many considerations probable. 



The part played by chlorophyll in photosynthesis has already been touched 

 upon. Remarkably little is known about chlorophyll itself. It has so far been 

 found impossible to extract it from the chloroplast without causing its decom- 

 position, and hence our ideas of its constitution, such as they are, are based upon 

 the examination of something differing in some not well-ascertained particulars 

 from the pigment itself. A remarkable relationship is known to exist between 

 the latter and iron, for unless this metal is supplied to a plant its chloroplasts do 

 not become green. But the condition of the iron in the plant is uncertain ; it 

 seems probable that it does not enter into the molecule of the pigment at all. 

 A remarkable series of resemblances between derivatives of chlorophyll and 

 derivatives of hsematin, the colouring matter of haemoglobin, has been brought to 

 light by the researches of Schunck and Marchlewski, which is very suggestive. 

 The same leaning towards iron is foimd in the two pigments, but in the case of 

 haematin our knowledge is further advanced than in that of chlorophyll. The 

 iron is known to be part of its molecule. It can by appropriate treatment be 

 removed, and a body known as hcematoporphyriji is then formed, which presents 

 a most striking similarity to a derivative of chlorophyll which has been named 

 phylloj)orphyrin. The two pigments are almost identical in their percentage 

 composition, the hsematoporphyrin containing a little more oxygen than the other. 

 Both seem to be derivatives of pyrrol. The most striking similarity between 

 them is their absorption spectra, their ethereal solutions both showing nine bands 

 of identical width and depth, those of hfematoporphyrin being a little more 

 towards the red end of the spectrum. Their solutions in alcohol and ether show 

 the same colour and the same fluorescence. Though they diifer in certain other 

 respects, notably the facility with which they form crystals, it is impossible to 

 deny that a close relationship seems probable. If this is established we may by 

 analogy perhaps learn something about the part played by iron in the action of 

 the chloroplast, which so far has proved as obscure as the relation of the metal 

 to the pigment. It is very suggestive to recall the resemblances between the two 

 pigments, the one playing so prominent a part in animal, the other in vegetable 

 life. Both are associated with a stroma of proteid, or possibly protoplasmic, 

 nature, in which a solution of the pigment is retained, apparently after the fashion 

 of a sponge. Both are concerned in metabolic processes in which gaseous inter- 

 changes play a prominent part. Both are in some way dependent on the presence 

 of iron for their individuality, even if iron is not actually present in the molecule 

 of both. The iron being removed, the derivatives which are found are almost 

 identical. Further researches may throw a light on this curious relationship, per- 

 haps showing that chlorophyll may enter into a combination with carbon dioxide 

 as hsematin does with oxygen. Such a combination might well be the precursor 

 of the decomposition of the carbon dioxide which has been already spoken of. 



We meet with another pigment in many plants the physiological significance 

 of which has in recent years begun to attract some attention. This is the red 

 colouring matter, anthocyan, apparently related to the tannins, which is developed 

 especially in the young leaves of shade-loving plants when they become exposed to 

 illumination exceeding the intensity which they normally encounter. The forma- 

 tion of this pigment is greatest in tropical plants, where it is found usually in 

 the epidermis of the young leaves, though in some cases it extends to the mesophyll 

 as well. The pigment seems in some way to be supplementary to chlorophyll, for 

 its absorption spectrum shows that it allows all the rays useful in photosynthesis 

 to pass through it. It is unlikely that it takes any share in photosynthesis. 

 Several theories have been advanced to explain its presence ; it may be simply to 

 protect the delicate cells from the destructive action of too intense light, or to 

 avert the evil of overheating from the solar rays. It has been suggested that 

 certain rays hinder the translocation of starch, and that the pigment shields the 

 cells from the incidence of such rays. Again the view has been advanced that the 

 red colour is important in accelerating the development of diastase from its 

 antecedent zymogen, which has been found to take place under the influence of 



