THE METABOLISM OF PLANTS. 163 



are equal, then it will be impossible to detect the substance 

 by micro-chemical methods, although the production of it 

 may be actively proceeding. It is in many cases possible so 

 to alter the conditions under which an organ is placed as to 

 disturb the relation between supply and demand (as we see in 

 Borodin's experiments mentioned in the last lecture) and thus 

 to bring about the accumulation of substances which cannot 

 be detected in it micro-chemically under normal conditions. 

 All that we can detect by micro-chemical methods is then 

 the excess of plastic material in the cell, and this we may 

 regard as temporary reserve-material. 



With this preliminary caution in our minds, we will discuss 

 first the distribution of the non-nitrogenous organic substance 

 formed by the leaves. This we have found to be starch, in 

 the vast majority of cases, and we have seen that it is de- 

 posited in the chlorophyll-corpuscles in the form of grains. 

 The proof that this starch is conveyed away from the leaves 

 is afforded by the observation of Sachs that if a leaf which 

 contains starch abundantly in its chlorophyll-corpuscles be 

 placed in the dark for some hours the starch-grains will be 

 found to have disappeared, and by those of Godlewski, Pfeffer, 

 and Morgen, that the starch-grains disappear from the chloro- 

 phyll-corpuscles of a leaf when it is exposed to light in an 

 atmosphere containing no carbon dioxide. We learn, then, 

 that if chlorophyll-corpuscles be placed under conditions 

 which prevent the continued formation of starch, no trace of 

 starch will usually be detected in them. It is obvious that the 

 starch-grains cannot be bodily conveyed as such from one cell 

 to another inasmuch as the walls of the mesophyll-cells, so far 

 as we know at present, are closed membranes; and even ad- 

 mitting, as was mentioned in a previous lecture (p. 23) that 

 the protoplasm of adjacent cells may possibly be continuous 

 through pores in the cell-wall, still the size of the starch-grains 

 is too great to allow of their passing through these pores. 

 The suggestion naturally occurs that the starch must pass 

 from cell to cell in solution, and that, since it is practically 

 insoluble in water, it must be converted into some substance 

 which is soluble. There are reasons for believing that this 



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