50 



GENERAL BOTANY 



a stream, and serve to indicate the rate and direction of cyto- 

 plasmic movement in the leaf cells of this plant. The green 

 pigment, chlorophyll, can be removed from the cytoplasmic 

 body of the plastids by treating the leaves with alcohol, when 

 the green pigment dissolves out, leaving the colorless plastid 

 behind in the cytoplasmic sac. We have already learned that 

 the green pigment enables the chloroplastids, with the aid of 

 the sun's energy, to build and store starch made from carbon 

 dioxide and water. The chloroplastids are thus of the greatest 

 importance in making the necessary starchy food for the plant. 



toplasmic sac 



Starch 



a Chloroplastid 



Starch 



Leucoplastid 



iromoplastids 



FIG. 28. Three kinds of plastids and the formation of starch grains 



a, a mature cell with chloroplastids embedded in the cytoplasmic sac; b, a single 



chloroplastid containing light-colored starch grains ; c, chromoplastids ; d, leucoplas- 



tids of begonia forming starch grains 



Leucoplastids (Fig. 28, d) are similar to chloroplastids except 

 that they lack the green pigment, chlorophyll, and are therefore 

 unable to manufacture starch from raw food elements such as 

 carbon dioxide and water. They are able, however, to transform 

 sugar into starch, and are therefore present in all such special 

 storage organs as potato tubers and seeds, as well as in ordinary 

 stems and leaves where starch is stored away from direct sun- 

 light. They are more difficult to demonstrate microscopically 

 than chloroplastids, and hence are less frequently seen than the 

 latter. Leucoplastids are often transformed into chloroplastids 

 by the secretion of green pigments when they are exposed to 

 sunlight. This is frequently observed when potato tubers are 

 uncovered and turn green in the part of the tuber which is 

 directly exposed to the sun. 



