DIGESTION AND ASSIMILATION 87 



183. Digestion. — The starch made by the leaf during the 

 daytime is present in the form of insoluble granules. In 

 order to be carried from the leaf to other parts of the plant 

 for purposes of storage or growth, it must be made soluble. 

 The starch of the leaves at night is converted into sugars 

 by the action of enzymes, or ferments, and is then conveyed 

 to other parts of the plant. This conversion is a process of 

 digestion. It is much like the change of starchy foods to 

 sugary foods by the saliva. 



184. After being changed to the soluble form, this material 

 is ready to be used in growth, either in the leaf, in the stem, 

 or in the roots. With other more complex products it is 

 then distributed throughout all of the growing parts of the 

 plant; and when passing down to the root it passes readily 

 through the inner bark, in plants that have a definite bark. 

 This gradual downward diffusion of materials suitable for 

 growth through the inner bark is the process referred to 

 when the "descent of sap" is mentioned. Starch and other 

 products are often stored in one growing season to be used 

 in the next season (Chapter VI). If a tree is constricted or 

 strangled by a wire around its trunk, the digested food can- 

 not readily pass down and it is stored above the girdle, caus- 

 ing an enlargement. 



185. Assimilation. — The food from the air and the nutrients 

 from the soil unite in the living tissues (see Photosynthesis, 

 178). The sap that passes upwards from the roots in the 

 growing season is made up largely of the soil-water and the 

 salts that have been absorbed in the diluted solutions. 

 We have found that this upward-moving water is conducted 

 largely through certain tubular cells of the young wood (153). 

 These cells are never continuous tubes from root to leaf; 

 but the water passes readily from one cell to another in its 

 upward course. 



186. The upward-moving water gradually passes t<> the 

 growing parts, and it comes in intimate contact with the 



