THE METABOLISM OF PLANTS. 145 



the plant. It was pointed out in a previous lecture (p. 15) 

 that, according to Schmitz and Strasburger and in harmony 

 with the older statements of Pringsheim, the cell-wall is pro- 

 duced by the actual conversion of a layer of protoplasm, and 

 we shall see hereafter that the same is asserted of the layers 

 of the starch-grains found in seeds, tubers, etc. Translating 

 this into chemical language we find it to mean that molecules 

 of protoplasm may undergo dissociation in such a way as to 

 give rise to molecules of carbohydrate among other products. 

 The conclusion to be drawn is, that the starch which is formed 

 in chlorophyll-corpuscles under the influence of light is also 

 the product of such a dissociation of protoplasm. Some 

 valuable support is afforded to this view by the observations 

 of Schimper and others that starch-grains may make their 

 appearance in chlorophyll-corpuscles under circumstances 

 which preclude any formation of them from carbon dioxide 

 and water. 



The non-nitrogenous organic substance which is first 

 formed in the chlorophyll-corpuscle from carbon dioxide and 

 water is, then, not starch, but a substance (possibly allied to 

 formic aldehyde) which goes to construct proteid, by com- 

 bining either with the nitrogen and sulphur absorbed in the 

 form of salts from the soil, or with the nitrogenous residues 

 of previous decompositions of proteid. The starch deposited 

 in the corpuscle is, however, the first visible product of the 

 constructive metabolism going on within it, for, unless proto- 

 plasm is being formed, no starch can be produced : it may 

 be regarded as a temporary reserve-material. 



Inasmuch as we now know that the visible deposits of 

 non-nitrogenous organic substance in the plant, in the form of 

 starch or cellulose, are derived from protoplasm, we can under- 

 stand how it is that the great differences in dry weight which 

 exist between plants which have and which have not been 

 supplied with assimilable nitrogen during their growth, are 

 brought about. The absence of assimilable nitrogen prevents 

 the formation of proteids in the latter case ; as a consequence, 

 cellulose and starch are formed only in small quantity, and 

 the plant remains small, its development being only due to 

 V. 10 



