LIVING PARTS OF THE STEM 113 



This loss of sugar would leave the sap rather more 

 watery than usual, and osmosis would carry it from the 

 growing wood to the leaves, w hile 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 (lowerless and very low and simple kinds. 



