72 ELEMENTS OF BOTANY. 



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

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

 those pertions 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, § 70. 



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 substance which the plant employs for this purpose, the 

 student will do well to spend the time which he devotes to 

 the study of storage of food in the stem to looking for starch 

 only. 



Cut thin cross-sections of twigs of any common hard-wood tree, in its 

 winter condition, moisten with iodine solution, and examine for starch 

 with a moderately high power of the microscope. Sketch the section, 

 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 to 

 deposit nourishment during winter to begin the growth of the 

 following spring. But in those plants which die down to the 

 ground at the beginning of winter the storage must be either 

 in the roots, as has been described in § 46, 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 



