CIIEMISTRT AND PHYSICS OF PLANTS. 81 



156. This decomposition and subsequent combination 

 take place only in the granules or masses of chlorophyll, 

 and only in sunlight. Those parts of ordinary plants which 

 are destitute of chlorophyll are entirely wanting in the 

 power of starch-making (assimilation), and likewise the 

 chlorophyll-bearing portions are unable to assimilate in 

 darkness. 



151. Digestion and Use of Starch. — In darkness the starch 

 which had previously formed in the chlorophyll-bodies un- 

 dergoes changes which render it soluble, allowing it to 

 diffuse to other parts of the plant with great freedom. The 

 nature of these changes appears to vary somewhat in dif- 

 ferent plants, but they consist essentially in the change of 

 the insoluble starch into a chemically similar but soluble 

 substance. Glucose (C,.^H,,0„), inulin (Cj^Hj^O,,), and 

 cane-sugar {C^^^fi^^ are the more common of the soluble 

 substances so formed, and one or other of these may fre- 

 quently be detected in the adjacent cells after the disap- 

 pearance of the starch from the chlorophyll. 



158. These diffusing assimilated matters are imbibed by 

 the protoplasm of the living tissues, and constitute its most 

 important food. In connection with the nitrates and sul- 

 phates, etc., also imbibed, it furnishes the materials for the 

 increase of protoplasmic substance in growing cells. 



159. The Storing of Eeserve Material. — In many plants 

 the surplus starch is stored up in one or more organs as re- 

 serve material; thus in the potato the starch formed in the 

 leaves in sunlight is, in darkness, transformed into glucose, 

 or a substance very nearly like it, and in this soluble form 

 it is diffused throughout the plant, and in the underground 

 stems (tubers) is again transformed into starch. So in the 

 case of many seeds a mass of reserve material is stored up. 



