78 WORK OF THE STEM 



sugar is set up from leaves to wood. The water is thrown off 

 in the form of vapor as fast as it reaches the leaves, so that they 

 do not become distended with water, while the sugar is changed 

 into cellulose and built into new wood cells as fast as it reaches 

 the region where such cells are being formed. 



Plants in general l readily change starch to sugar, and sugar 

 to starch. When they are depositing starch in any part of the 

 root or stem for future use, the withdrawal of sugar from those 

 portions of the sap which contain it most abundantly gives rise 

 to a slow movement of dissolved particles of sugar in the direc- 

 tion of the region where starch is being laid up. 



91. Storage of food in the stem. The reason why the plant 

 may profit by laying up a food supply somewhere inside its 

 tissues has already been suggested (Sec. 33). 



The most remarkable instance of storage of food in the stem 

 is probably that of sago palms, which contain an enormous 

 amount, sometimes as much as eight hundred pounds, of starchy 

 material in a single trunk. But the commoner plants of tem- 

 perate regions furnish abundant examples of deposits of food 

 in the stem. 



92. Storage in underground stems. The branches and trunk 

 of a tree furnish the most convenient place in which to deposit 

 food during winter to begin the growth of the following spring. 

 But in those plants which die down to the ground at the begin- 

 ning of winter the storage must be either in the roots or in 

 underground portions of the stem. 



Eootstocks, tubers, and bulbs seem to have been developed 

 by plants to answer as storehouses through the winter (or in 

 some countries through the dry season) for the reserve materials 

 which the plant has accumulated during the growing season. 

 The commonest tuber is the potato, and this fact and the points 

 of interest which it represents make it especially desirable to 

 use for a study of the underground stem in a form most highly 

 specialized for the storage of starch and other valuable products. 

 1 Not including most of the spore plants. 



