HOW PLANTS GET THEIH CARBON FOOD. 79 



Demonstration 26. 



146. Form of starch grains. — Where starch is stored as a reserve mate- 

 rial it occurs in grains which usually have certain characters peculiar to the 

 species of plant in which they are found. They vary in size in many dif- 

 ferent plants, and to some extent in form also. Scrape some of the cut sur- 

 face of the potato tuber into a pulp and mount a small quantity in water, or 

 make a thin section for microscopic examination. We find large starch grains 

 of a beautiful structure. The grains are oval in form and more or less irregular 

 in outline. But the striking peculiarity is the presence of what seem to be 

 alternating dark and light lines in the starch grain. The lines form irregu- 

 lar rings, which are smaller and smaller until we come to the small central 

 spot termed the " hilum " of the starch grain. It is supposed that these ap- 

 parent lines in the starch grain are caused by the starch substance being 

 deposited in alternating dense and dilute layers, the dilute layers containing 

 more water than the dense ones ; others think that the successive layers 

 from the hilum outward are regularly of diminishing density, and that this 

 gives the appearance of alternating lines. 



147. Necessity of carbon food for plants. — The starch 

 formed by plants is one of the organic substances manufactured 

 by plants. It is the basis for the formation of other organic sub- 

 stances. Starch contains carbon, hydrogen, and oxygen, in the 

 proportion of 6 molecules of carbon, io molecules of hydrogen, 

 and 5 molecules of oxygen (C 8 H ln 5 ). The water in the starch 

 is in the proportion of 2 molecules of hydrogen to i molecule 

 of oxygen (H,0). For this reason it is called a carbohydrate. 

 The most important carbohydrates in plants are starch, the 

 sugars, and cellulose, the latter substance, or modifications of 

 it, forming the cell walls of plants. Without carbon-food 

 green plants cannot make any appreciable increase in plant 

 substance, though a considerable increase in size of the plant 

 may take place (see paragraph 194). Chlorophylless plants, like 

 the fungi and certain parasitic or saprophytic (as the Indian- 

 pipe, certain of the orchids, etc.) angiosperms, derive their 

 carbon-food from the carbohydrates manufactured by the green 

 plants. Animals also derive their carbohydrates through the 

 medium of the green plants, either directly or indirectly. 



Note. For further experiments and discussion of this subject see the 



author's larger "Elementary Botany." 



