LIVING PARTS OF THE STEM 113 
This loss of sugar would cause a flow of rather watery 
sap to take place more rapidly than usual from the grow- 
ing wood to the leaves, while at the same time a slow 
transfer of the dissolved sugar will be set up from leaves 
to wood. ‘The water, as fast as it reaches the leaves, will 
be thrown off in the form of vapor, so that they will 
not become distended with water, 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 dis- 
solved particles of sugar in the direction of the region 
where starch is being laid up. 
121. 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. 91). 
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 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 student will do well to spend time in looking 
for starch only. 
1 Not including most of the flowerless and very low and simple kinds. 
