82 ESSENTIALS OF BOTANY 



become distended, while the sugar will be 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 ^ 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 direction of the region where 

 starch is being laid up. 



96. 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 (Sect. 76). 



The most remarkable instance of storage of food in the 

 stem is probably that of sago-palms, which contain an enor- 

 mous amount, sometimes as much as 800 pounds, of starchy 

 material in a single trunk. But the commoner plants of 

 temperate regions furnish plenty of examples of deposits 

 of food in the stem. As in the case of seeds and roots, 

 starch constitutes one of the most important kinds of this 

 reserve material of the stem, and since it is easier to detect 

 than any other food material which the plant stores, the stu- 

 dent will do well to spend time in looking for starch only. 



Cut thin cross-sections of twigs of some common deciduous tree 

 or shrub, in its early winter condition, moisten with iodine solution, 

 and examine for starch with a moderately high power of the micro- 

 scope. Sketch the section with a pencil, coloring the starchy por- 

 tions with blue ink, used with a mapping pen, and describe exactly 

 in what portions the starch is deposited. 



97. Storage in Underground Stems The branches and 



trunk of a tree furnish the most convenient place in which 



J Not including most of the flowerleas and very low and simple kinds. 



