152 LECTURE IX. 



substance de novo can take place in the plant. We have now 

 to ascertain what is the part played by the green colouring- 

 matter, the chlorophyll, in the process. 



Carbon dioxide is absorbed and oxygen is exhaled by some plants 

 and parts of plants which are not green, such, for instance, as the brown 

 Algae (Fucoideae), the red Algae (Florideae), and the leaves of the Copper 

 Beech. Chlorophyll is, however, present in all these cases, but the green 

 colour is not perceptible on account of the presence of other colouring- 

 matters. 



It was pointed out long ago by Draper that the chlorophyll-corpuscles 

 of etiolated plants, of plants, that is, which have grown in darkness, can 

 absorb carbon dioxide and exhale oxygen under the influence of light, an 

 observation which has been recently confirmed by the researches of 

 Engelmann. These corpuscles are not green, but yellow ; they contain a 

 colouring^jftatter known as etiolin, which is doubtless closely allied to 

 chlorophyll and is converted into chlorophyll when the etiolin-corpuscles 

 are exposed to light. It appears from the above-mentioned observations 

 that etiolin plays the same part in relation to the decomposition of carbon 

 dioxide as chlorophyll: and Engelmann is of opinion that this is true 

 also of the colouring-matters of the Algae (phycoxanthin, phycocyanin, 

 phycoerythrin). 



In the simpler unicellular plants, Haematococcus for ex- 

 ample, the chlorophyll is distributed throughout the proto- 

 plasm of the cell, but in all the higher forms it is confined to 

 specialised portions of the protoplasm, usually somewhat oval 

 in outline and discoid or lenticular in form, the chlorophyll- 

 corpuscles with which we have already become so familiar. In 

 certain Algae, the Conjugatae, a group to which the Desmids 

 and Spirogyra and its allies belong, these chlorophyll-corpus- 

 cles are imbedded in plates of green protoplasm, the chloro- 

 phyll-bodies, but it is apparently only in the corpuscles them- 

 selves that the formation of starch takes place. 



We may begin our study of the function of chlorophyll by 

 saying that the absorption of carbon dioxide, the evolution of 

 oxygen, and the formation of new organic substance, are 

 effected entirely and solely by .chlorophyll-corpuscles, for we 

 may be permitted to regard the green protoplasm of a unicel- 

 lular Alga as constituting one large chlorophyll-corpuscle. 

 These processes go on in the chlorophyll-corpuscle quite inde- 

 pendently of the uncoloured protoplasm of the cell, 'for Engel- 



