22 



GROWTH AND WORK OF PLANTS 



30. The starch grains are surrounded by a thin coating of 

 cellulose, a substance similar to that of which the cell walls of 

 plants are made. This must be dissolved before the plant can 

 use the starch for food. The plant dissolves it by the use of 

 certain cellulose ferments (paragraph 208) manufactured by it, 

 and then the starch is dissolved by the diastase ferment (para- 

 graph 184). A similar diastase is present in the saliva of man, 



but this will not act on the 

 starch grains unless the 

 cellulose coat is broken, or 

 dissolved by some ferment. 

 Heat causes the starch 

 grains to swell. If they are 

 mixed with water, gentle 

 heat changes the grains to a 

 paste. Dry heat causes the 

 starch grains to burst. This 

 is why thorough cooking of 

 starchy vegetables and 

 fruits makes them easier of 

 digestion. 



31. Corrosion of starch 

 grains during germina- 

 tion. Since plants cannot 

 absorb solid particles of 

 food, the embryo in the seed 

 cannot use the starch for 

 food until after it is dis- 

 solved. The embryo has the power to excrete a juice or ferment 

 (diastase) which acts on the starch grains, dissolving them and 

 changing them to a sugar. During this process the starch grains 

 become corroded. To see the corroded starch grains, take some 

 of the endosperm of the grain of corn at different stages of 

 germination and examine under the microscope. In comparing 

 the appearance of these starch grains with those in ungermi- 

 nated seeds, the results of the corrosion are clearly seen. 



Fig. 28. 



Cell of endosperm of Indian corn, containing 

 polygonal starch grains, separated by thin plates 

 of protoplasm. In the figures a to g, the starch 

 grains, taken from a germinating Indian corn 

 grain, are becoming dissolved and disintegrated. 

 After Sachs. 



