80 THE MAKING OF THE LIVING MATTER 



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 from the air and that 

 from the soil unite in the living tissues (see photosyn- 

 thesis, 162, 163) . 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. This up ward -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, par- 

 ticularly in the leaves, it meets the products of assimilation 

 from the leafy parts. Under the influence of the living 

 matter of the plant, this product from the leaves first 

 forms combinations with the nitrogen. A substance more 

 complex than sugar is then formed, and gradually com- 

 pounds are formed which contain sulfur, phosphorus, potas- 

 sium, and other elements, until finally protoplasm is manu- 

 factured. 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 proto- 

 plasm is called assimilation. 



172. RESPIRATION. Plants need oxygen for respira- 

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



