CHEMISTRY 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. 
157. 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,,O,,), inulin (C,,H,,O,,), and 
cane-sugar (C,,H,,O,,) 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 Reserve 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. ‘“o in the 
case of many seeds a mass of reserve material} tored up, 
