PROTOPLASM AND PLANT-GELLS. 



15 



23. Starch is a plant-food. It is produced by the green 

 protoplasm for the nourishment of the plant. As it forms 

 only in light, during the day it accumulates, but at night 



Fig. 9. 



Fio. 10. 



Fig. 9.— a few cells of the seed of a Pea, showing large starch-graina 

 XSt) and the little granules of aleurone (a). At i, i, are shown intercellu- 

 lar spaces. Magnified 800 times. 



Fig. 10.— Leucoplasts (I) and young starch-grains of an orchid (Phajus). 

 Magnified 540 diameters. {From Strasburger.) 



by the continued activity of the plant it is greatly dimin- 

 ished. Whenever there is more made than the plant re- 

 quires, the surplus is stored by the leucoplasts in certain 

 cells for future use. 



Practical Studies. — (a) Scrape ofE a little of the substance ol the 

 cut surface of a potato-tuber. Mount in water and examine under 

 the microscope, using the \ objective. Note the ovate starch-grains, 

 which are concentrically striated. Now add a small drop of iodine 

 and note the blue coloration, which becomes purple or purple-black 

 if much iodine is used. 



(b) Make an extremely thin slice of the potato-tuber and treat as 

 before, so as to observe starch-grains in the cells. By staining such 



