9 6 



PLANT BIOLOGY 



Distribution of the Digested Food. 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 seems to pass 

 more readily through the inner 

 bark y in plants which have a defi- 

 nite bark. This gradual down- 

 ward diffusion through the inner 

 bark of materials suitable for 

 growth is the process referred to 

 when the " descent of sap " is men- 

 tioned. Starch and other products 

 are often stored in one growing 

 season to be used in the next sea- 

 son. If a tree is constricted or 

 strangled by a wire around its 

 trunk (Fig. 118), the digested food 

 cannot readily pass down and it is stored above the girdle, 

 causing an enlargement. 



Assimilation. The food from the air and that from the 

 soil unite in the living tissues. The "sap" that passes 

 upwards from the roots in the growing season is made up 

 largely of the soil water and the salts which have been 

 absorbed in the diluted solutions (p. 67). This upward- 

 moving water is conducted largely through certain tubular 

 canals of the young wood. These cells are never continu- 

 ous tubes from root to leaf; but the water passes readily 

 from one cell or canal to another in its upward course. 



The upward-moving water gradually passes to the grow- 

 ing parts, and everywhere in the living tissues, it is of 



FIG. 118. TRUNK GIRDLED 

 BY A WIRE. See Fig. 85. 



