84 PLANT PHYSIOLOGY AND ECOLOGY 



away to the stem, flowers, and roots. Starch appears in the chlo- 

 roplasts only when the glucose is formed more rapidly than it is re- 

 moved. This is usually the case in the sunlight, and starch accumu- 

 lates throughout the day. When photosynthesis ceases at night, 

 the starch in the plastid is converted into glucose by the chemical 

 addition of a molecule of water to each molecule of starch, and the 

 glucose is transported in solution to the different parts of the plant. 

 The removal, or translocation, of the glucose normally takes place 

 during day and night, but, since this substance is usually made about 

 ten times faster than it can be removed, translocation is most active 

 at night. This movement of the soluble food is largely caused 

 by differences in osmotic pressure between the various cells. Its 

 direction is determined by the greatest use, the movement in 

 general being toward the organ or tissue that is most actively 

 assimilating or storing the food. In the chlorenchym as well 

 as in the parenchyma of the stem and root, the glucose moves 

 by ordinary diffusion. In its passage from the leaves to the 

 other organs of the plants, it follows a more definite pathway. 

 This is regularly constituted by the sieve tissues of the fibro- 

 vascular bundle, though it is not impossible that glucose should 

 reach the root by diffusion through the parenchyma of the stem. 

 This is a slow process, however, and by far the greater amount 

 of glucose must reach the various organs by way of the sieve 

 tissue. Thus, the fibrovascular bundles constitute a complete 

 system for the transport of the foods made b}^ the leaves, as well 

 as for that of the water and solutes absorbed by the roots. 



Experiment 26. Translocation. Select a vigorous sunflower that has 

 been in the sunshine all day. Cut a leaf from it and put the leaf away 

 in a moist chamber. Sever the midrib or primary veins of a second 

 leaf, cutting through almost to the upper epidermis. Early the next 

 morning, remove the severed leaf and an uninjured one from the plant. 

 Cut cross-sections of the three leaves, and explain the differences in 

 starch content. 



106. Storage of food material. The storage of glucose in the 

 form of starch may take place in practically any part of the plant. 

 Except for the temporary formation of starch in the chlorop lasts, 

 storage in the green leaf is rare. It occurs very often in seed- 

 leaves, and frequently in seeds, stems, and roots. In all cases 

 the method by which the material is stored is the same. It is 



