80 THE MAKING OF THE LIVING MATTEK 



tributed 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, in plants which have a defi- 

 nite 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 cannot readily pass down and it is stored 

 above the girdle, causing an enlargement. 



170. ASSIMILATION. The food obtained from the air and 

 that from the soil unite in the living tissues. The sap 

 which is constantly passing upward from the roots during 

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

 and the salts which have been absorbed in diluted solu- 

 tions. This upward-moving water is conducted largely 

 through certain tubular cells of the young wood. These 

 cells are never continuous tubes from root to leaf; but the 

 water passes readily from one cell to another in its upward 

 course. 



171. The upward -moving water gradually passes to the 

 growing parts, and everywhere in the living tissues it 

 meets the liquid product returning from the leafy parts. 

 Under the influence of the living matter of the plant, this 

 product from the leaves first selects the nitrogen. A sub- 

 stance more complex than sugar is then formed, and grad- 

 ually compounds are formed which contain sulfur, phospho- 

 rus, potassium, and other elements, until finally protoplasm 

 is manufactured. Protoplasm is the living matter in plants. 

 It is in the cells, and is usually semi-fluid. Starch is not 

 living matter. The complex process of building up the 

 protoplasm is called assimilation. 



172. RESPIRATION. Plants needs oxygen for respira- 

 tion just as animals do. We have seen that plants need the 



