STARCH-MAKING AND DIGESTION 



79 



different kinds are known. They are peculiar in that they 

 seem to induce chemical changes in other siibstances, without, 

 hoivever, undergoing any changes themselves. As a result of 

 this peculiarity a comparatively large 

 amount of material may be made to 

 undergo chemical change through the 

 activity of a very small amount of enzym. 

 113. Food transportation. The cells 

 of the chlorophyl-bearing tissues con- 

 tain diastase and other ferments. In 

 the light, some of the sugar that is 

 formed passes out of the pulp cells and 

 is carried down in the bast, or phloem 

 tubes (see p. 176). But under favorable 

 conditions for food-making the sugar is 

 manufactured faster than it can be car- 

 ried away. Most of it is then converted 

 into starch, which is insoluble. In this 

 way it accumulates in the cells during 

 the day. When darkness sets in, a dia- 

 static action converts the starch into 

 sugar, and this is then carried down into 

 the stem or roots (see diagram, Fig. 26). 



This explains why. leaves that are full of 

 starch in the late afternoon show no signs 

 of starch very early in the morning. As the 

 morning light increases in intensity, starch 

 is accumulated, and in the afternoon the 

 cells are again full. From this we can also 

 understand the presence of starch in potato 

 tubers and in other organs that do not contain 



chlorophyl. The starch is formed in the cells of the tuber by the action 

 of a ferment upon sugar. The sugar is brought from the leaves, pass- 

 ing at first from cell to cell by osmosis, then in the sap by "way of the 

 bast tubes. In the root or tuber the sugar passes from the vessels to 

 the wood or bark cells by osmosis, and is then converted into starch. 



FIG. 26. Starch in light 

 and darkness 



During the daytime the plant 

 manufactures carbohydrate in 

 the leaves, receiving a steady 

 supply of water from the soil. 

 In the dark the starch is 

 changed into sugar and there 

 is a stream of sap running 

 downward into the roots or 

 underground stems, where 

 the surplus is accumulated 

 as starch 



