40 PLANT LIFE. 



or strise, which originate from a point called the hiluni, itself 

 the starting-point of the formation of the starch grain. The 

 lines of stratification are due to the alternation of more and 

 less watery layers of starch. The grains consist of starch, 

 water, and a very minute proportion of mineral matter. 

 The starch has the same chemical composition as cellulose, 

 CeH^Oj, and consists of two substances, granulose, and 

 starch-cellulose; the latter forms only a very small percent- 

 age of the structure. Starch becomes blue when treated 

 with a solution of iodine ; this change is used as a test for 

 its presence. In reality the granulose is only coloured blue 

 by the iodine. 



Although it has been stated that chlorophyll grains form 

 starch from carbon dioxide and water under the influence of 

 light, yet it is not considered that the conversion of these 

 two substances into starch is direct, but that this substance 

 is in all instances formed from protoplasm. In the internal 

 dark parts of plants starch is formed by amyloplasts. These 

 bodies closely resemble chromoplasts, in fact the conversion 

 of the former into the latter under the influence of light has 

 been directly observed. The difference in function between 

 the two may be stated as follows. In both cases the starch 

 formed is the product of decomposed protoplasm ; in the 

 chlorophyll grain exposed to light, the process begins with 

 chemical changes produced on inorganic substances, as water, 

 carbonic dioxide, and salts ; whereas in amyloplasts it begins 

 with organic substances, and light is not essential. 



Cell-sap is always present in young cells, and remains so 

 long as the cell is doing physiological work. Many organic 

 substances formed by the protoplasm remain in solution 

 in the cell-sap, as vegetable acids, malic (C4H6O5), citric 

 (CfiHsO,); cane-sugar (CeH^Oj), grape-sugar (C^H^O^); 



