148 LECTURE IX. 



do and which do not contain chlorophyll is the fact which we 

 are now considering, namely, that plants which contain chloro- 

 phyll can, under the influence of light, construct non-nitro- 

 genous organic substance out of carbon dioxide and water. 

 A plant which does not contain chlorophyll cannot do this ; 

 it is therefore necessary that it should be supplied from with- 

 out with food in which carbon is already in organic combina- 

 tion (see p. 124). In connexion with this we may mention 

 the fact that no starch is ever found in the tissues of Fungi, 

 though it is difficult to say why this should be. It is very 

 remarkable that although the protoplasm of these plants 

 undergoes dissociation in such a way as to produce cellu- 

 lose and other carbohydrates, it does not undergo dissociation 

 so as to produce the carbohydrate starch. 



2. The Formation of Nitrogenous Organic Substance. 



We learned in the last lecture that plants can only avail 

 themselves of nitrogen in the combined form, either as in- 

 organic salts or as certain organic compounds, for the purposes 

 of their constructive metabolism. With regard to the con- 

 ditions of the formation of nitrogenous organic substance in 

 the plant, we know, from our study of Yeast, that it is inde- 

 pendent of the presence of chlorophyll, and it apparently goes 

 on as well in the absence as in the presence of light. In these 

 respects it contrasts in a marked manner with the formation 

 of non-nitrogenous organic substance. All that we shall have 

 to say about it will refer to the probable mode and the 

 probable place of its occurrence, and unfortunately we are 

 compelled, in the absence of definite information on these 

 points, to confine ourselves to probabilities. 



We know that green plants form non-nitrogenous organic 

 substance, that plants destitute of chlorophyll, parasites or 

 saprophytes, absorjD substances of this nature, and that all 

 plants absorb inorganic compounds of nitrogen, such as ni- 

 trates and salts of ammonia, as well as sulphates : here then 

 are the materials for the formation of nitrogenous organic 

 substance. But though we know what are the necessary 



