84 



HISTOLOGY AND PHYSIOLOGY. 



flowers differ from chlorophyll in that the former appear to 

 be indepeiKlent of light for their production. Flowers ex- 

 panding in the dark have their natural colors. 



107. The starch grains arise in the chlorophyll as a 

 product of assimilation. They may be of various shapes, 

 as lenticular in Wheat grains, oval in the Potato, many- 

 sided in Indian Corn, elongated, with enlarged ends, in the 

 milky juice of Euphorbia splendens. The average size and 

 shape of the grains are characteristic for the different 

 plants; and, therefore, when starch is examined microscop- 

 ically, it can be determined from what plant it was ob- 

 tained. Some grains have one 

 nucleus, and are, therefore, sim- 

 ple (Fig. 191) ; others have two 

 or more nuclei, and are com- 

 pound (Fig. 192). Surrounding 

 the nucleus are concentric lay- 

 ers, light and dark alternating, 

 as in the cell-wall, and due to 



the same cause, namely, greater and lesser amounts of 

 ■water contained. The molecular structure is the same as 

 that of the cell-wall, and growth takes place by intussus- 

 ception, or intercalation of particles between those already 

 existing. Chemically, starch resembles sugar and cellulose, 

 and is composed of carbon, hydrogen, and oxygen. It 

 turns a deep blue color when a solution of iodine is 

 applied to it. 



108. After starch has been formed in the chlorophyll- 

 bearing cells, it becomes dissolved, and is either consumed 

 at once in the formation of vegetable fabric, or is trans- 

 ported to some other part of the plant, where It reappears 



')(4 3o 



191 



192 



Figs. 191, 192. Starchgrains from a Potato: 191. Simple; 192. Compound grains. 



