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 
1 Not including most of the flowerless and very low and simple kinds. 
