WORK OF THE SHOOT 



87 



influence is withdrawn, and photosynthesis is not going on, 

 the ferment diastase is actively at work digesting the starch 

 in the leaves, and the sugar thus formed is drained away 

 to other organs. On the return of sunlight, the leaf is again 

 ready to continue the work of photosynthesis. We thus see 

 how a plant utilizes the alternating periods of day and night. 



Starch formed from sugar in the dark by leucoplasts. 

 By osmosis the sugar solution so formed in the sap is 

 transferred from cell to cell and carried downwards through 

 the leaf-stalk to the stem, and even to the root ; and in 

 plants developing their seeds it is conveyed to the cotyle- 

 dons, e. g. the Pea and Bean, or to 

 the endosperm, as in the Wheat. 

 On reaching these organs the sugar 

 is once more converted into starch 

 grains ; thus the surplus organic 

 food is transferred to storage organs 

 and there laid by until required. 



In the inner parts of plants, 

 where light cannot penetrate, are 

 often found small, rounded, colour- 

 less bodies having the same origin 

 as chlorophyll corpuscles and 



known as leucoplasts (Gr. leukos = white). Owing to the 

 absence of chlorophyll they are unable to manufacture 

 starch from carbon dioxide and water, but can build up 

 starch grains from sugar carried to them from the green 

 parts. In this way starch grains arise in parts that grow 

 in the dark (Fig. 53). If, however, tissues containing 

 leucoplasts, e.g. potato tubers, are exposed to light, the 

 leucoplasts develop the green pigment and become chloro- 

 phyll corpuscles. Thus, by the action of green corpuscles, 

 solid food-substances are formed during sunlight. These 

 are rendered soluble by ferments and can be transferred 

 to organs where they may be reconverted by white 

 corpuscles into solid food-reserves. 



Fig. 53. Cells of the 



Potato containing 



Starch Grains. 



